Xanathar’s Guide to Everything has guidelines as to how many and what kinds of magic items a party should have at a given level. “Awarding Magic Items,” (XGtE, 135-136)

Based on the tables on that page, a typical 10th Level party is expected to have 37 minor items and 8 major items.

Minor Items (DMG Magic Item Tables A - E): 16 common minor items, 14 uncommon minor items, 6 rare minor items, 1 very rare minor item

Major Items (DMG Magic Item Tables F - I): 7 uncommon major items, 1 rare major item

Sounds about right. I’m 50+ sessions in and they are about to get to the Amber Temple. If I rush the final act of the module, I might finish by the end of the year! Ha!

Yup. It's called "homebrew" and DMs do it all the time.

Moreover, DMs have been known to use books of 5th Edition monsters, races, classes and subclasses from other publisher whenever they feel like it. Kobold Press's Tome of Beasts, MCDM's Flee Mortals, and EN Publishing's Monsterous Menagerie are just the most well-known books of monsters for 5e from publishers other than Wizards of the Coast.

Creating homebrew races, classes and subclasses can be done, but probably shouldn't. I'm a DM, but that doesn't mean I have a clue about how to balance racial traits, class, and subclass features to make something new that is balanced against the rest of the material already exhaustively play-tested by professional game designers.

Edit: that being said, there are certainly races, classes, and subclasses by publishers that are not Wizards of the Coast which have been designed and play-tested by professional game designers. These you would not find in Dungeons & Dragons books, but maybe in a book by another 5e publisher.

Falling actually is actually that fast. According to one calculator I found, it takes under six seconds to fall 500 feet. So for game purposes, to keep it simple, falling up to 500 feet happens “instantly” though there is just enough time for a friend to cast feather fall if they are close enough.

These rules for falling come from Xanathar’s Guide to Everything, and if your DM is using those, then yes: “If you use the rule for rate of falling…, a flying creature descends 500 feet on the turn when it falls, just as other creatures do. But if that creature starts any of its later turns still falling and is prone, it can halt the fall on its turn by spending half its flying speed to counter the prone condition (as if it were standing up in midair).” XGtE, pg 77

Your “dive bomb” tactic is a reasonable interpretation of RAW IF your DM is using Xanathar’s rules, which he can pick and choose from as he sees fit. They are optional rules, after all.

Grappled is great! When I run monsters, and they grapple PCs, the monster can then move with the PC in tow at half speed. PCs are all like, "Only grappled? That's not too bad." Until the wolf drags them from the back line into melee with the rest of the pack, or the vampire spawn spider climbs with them 15 feet up the castle wall before sinking their fangs deep into their throat.

Running a PC, the right grapple at the right time can end a combat encounter quick and easy, preserving resources for later fights. The Barbarian can just grapple and shove enemies into chasms, off boats, into lava, and into spirit guardians.

To make encounter's more difficult or challenging for your PCs, there are several different parameters you can adjust (mid combat if necessary).

The number of monsters. The PCs thought they were hunting a single chimera, but after they engage it, they discover it was the favored pet of a half-ogre leading a small clan of goblins. They show up at the top of the second round and roll initiative.

Monster damage. Make sure you are choosing a monster or group of monsters that can reduce a single NPC to 0 HP in the first round of combat, and then, if you get the chance - REDUCE A PC TO 0 HP IN THE FIRST ROUND OF COMBAT. You know how many HP your PCs have. You know how much damage the monsters do on average (use average damage - it makes combat faster and more predictable). So pick a monster or group of monsters that can KO the paladin in round one. On the other hand, if you have a monster that can hit the entire party with a devastating AoE (like a dragon) on their first turn, you should do that. Have the dragon use its breath weapon on the party, then fly 15 feet above the wizard and use its legendary tail attack until the wizard get's knocked out.

Monster HP. It sounds like you shouldn't mess with HP any more. You've already maxed it out. I'm surprised your combats ever end. But in general, increasing monster HP does not, in my experience, make for better, harder combat. It just makes for longer combat. Much better to make your monster's hit harder and more often.

The Environment. Geysers, lava flows, rock slides, collapsing ceilings and floors, anti-magic zones, wild magic zones, advantageous positioning and terrain for your monsters, etc. You can actually do something unexpected and surprise your players just by collapsing a bit of the ceiling on them.

Let's be clear. If every combat is "balanced" and "just challenging enough," your game will be a boring slog. Some combats should be easy and fun, others should be obviously too much for the PCs and fleeing is the only option. Some should be hard. Boss battles should be deadly. Variety is the spice of life.

Theater of the mind for exploration and simple combat.

For example, when I ran Lost Mine of Phandelver, I prepared drawn maps on 1-inch grid paper only for the rooms in which combat would happen. If there was no combat, then no map necessary.

I’m running Curse of Strahd right now. In the castle, one player has taken it upon himself to map the dungeon on graph paper meticulously as we go to help the party navigate its labyrinthine passages. Cool. I still only draw out a map on a dry erase battle map or 1-inch grid paper only when there is a need for it.

There’s a more recent Starter Set with the adventure Dragons of Stormwreck Isle. It’s a 1st - 3rd Level adventure. I can imagine running the whole thing in 4 - 6 sessions depending on your group. As far as beginner adventures goes, it’s really good.

I know I’m being a huge rules lawyer here but… heat metal works on a “manufactured metal object,” and an iron golem is a creature (yes, a construct, but not an object). RAW you cannot cast heat metal on an iron golem. If it is wielding a “manufactured metal object” you cast on that and achieve the same effect. RAW, an iron golem wields a sword.

As to your other point, the creature only makes the CON save at all if it takes damage from the red hot metal object. The iron golem would take no damage because it is immune to fire damage, would not even have to make the CON save, and would not have to attack with disadvantage.

TL;DR: RAW, you can’t cast heat metal on an iron golem (or any other creature), but you can cast it on its sword to achieve OP’s desired result.

Various different events spread throughout the module might trigger an invitation to Ravenloft for dinner.

For example: in the “Developments” section at the end of the Feast of Saint Andral at the end of the Vallaki chapter, “If the characters thwart the attack on the church and protect Father Lucian, Strahd pays a visit to Wachterhaus (area N4) and there composes a letter, which he asks Lady Wachter to deliver to the characters. The letter is written in Strahd's hand and extends an invitation to the characters to come to Castle Ravenloft. Lady Wachter orders her spy, Ernst Larnak, or one of her sons to take the letter to the characters. If the characters open and read it, show the players "Strahd's Invitation" in appendix F. If the characters head toward the castle, they have no threatening random encounters on the way.”

Paragraphs like this are sprinkled throughout the module:

In Vallaki as detailed above

If the PCs allow Ireena to leave with Sergei via the pool in Krezk

If the PCs form an alliance with Ezmerelda as detailed in Van Richten’s Tower

I think that’s it though.

But the idea is, when Strahd begins to take a greater interest in the PCs, he invites them to the Castle. As written, this is a trap. He invites the characters to dinner, then traps them in the castle. They must either escape with their lives or hunt him down and kill him.

As a player, I love dungeon crawls. Moving from room to room, checking for traps, looting treasure chests, and killing monsters. When people talk about the “exploration pillar” of D&D, dungeons are where it just shines.

As a DM, I think about what kind of dungeon experience I would like to have as a player. Then I put that stuff in there.

I enjoy dungeons that have a story to tell. This dungeon had a history. Something dramatic happened here. Throughout this place, the PCs discover journals, ledgers, scraps of paper, or ghosts and spirits that deliver cryptic information in bits and pieces. By the time they win through the dungeon, they have discovered its whole story. Hopefully this story relates in some way to the BBEG of the dungeon and they find themselves excited to defeat it because they know its history and it is horrible.

I enjoy dungeons with social encounters. Those three ogres are not initially hostile. They are arguing amongst themselves about whose mother was the ugliest. The clan of kobolds in this part of the dungeon can be bargained with and placated in some way. The evil archmage attempts to bribe the PCs to just walk away and leave her to seek the secrets of lichdom in peace.

I love it when there is something weird and unexpected to discover. The PCs step through a door into an efreeti’s living room in the city of brass. Maybe they stumble into a zone of wild magic! Or they experience hallucinations as they wander through a fungal cavern. Or they discover rival adventurers paralyzed and poisoned wrapped in spider webs.

I usually go with theater of the mind until combat or precise positioning becomes important. On a VTT, I use one large token to represent “the party,” and move it around on the map.

Give it a try with the new Starter Set with the adventure Dragons of Stormwreck Isle. It doesn't get as much love as Lost Mine of Phandelver, but that's just because Lost Mine has been around for 10 years and everyone loved it as the first published 5th edition adventure. Dragons of Stormwreck Isle is a great starter adventure (1st - 3rd Level). I don't know what it costs on Roll20, but the box set is available on Amazon for like $14 US.

But it is not a long-term campaign. I cannot imagine it taking more than 5 or 6 sessions to run. You could then transition into Dragon of Icespire Peak or (if you are very ambitious) Curse of Strahd or Storm King's Thunder. The recent Planescape adventure, Turn of Fortune's Wheel begins at 3rd Level, but that set is pricey.

Grog

Just for a moment, take your DM hat off and think like a player. What would you, as a player, think and do if you witnessed an assassin killing an NPC. Wouldn’t you assume that you would be able to act in some way to prevent it? How would you feel if you said something like, “I have 45 feet of movement, and Ki to spare. I want to move, used step of the wind to Dash, then use two unarmed strikes to try to stun the assassin,” and your DM said, “No. you can’t do that. This is a cut scene. This NPC must die.”

In the example situation, the DM should reply, “Ok. Roll initiative!”

I’ve played in too many sessions in which something happened and we were powerless to do anything about it because the DM had a story in mind and a plot point they needed to do. It leaves a bad taste in my mouth every time. As a player, this kind of thing just feels awful.

Knowing this, as a DM, I avoid at all costs.

However, the bad guys often have resources and opportunities the PCs do not. This assassin might have a powerful poison as you describe. A popular actual play show even introduced the idea of a poison that negates resurrection magic.

Even if it is unlikely that the PCs win, save the NPC, and kill or chase off the assassin, let them act! And be prepared for the chance that they actually win. Maybe the assassin downs or kills one of the PCs and escapes. Now the party has a villain they hate who has defeated them once. Revenge will be even sweeter.

I rolled on the random location table and got some great results. These are so good, I’ll just add them in to the adventure every time I run it. 1. Cryovain appears as the PCs are finishing up one of the first round of quests, ideally at the end of the Dwarven Excavation when the orcs attack. He swoops in freezes a bunch of orcs, picks one up and flies off to eat in peace. 2. Cryovain attacks Phandalin as the PCs are returning, preferably around 3rd or 4th Level, but before they’ve gone to Dragon Barrow. He freezes a field of livestock, but allow the PCs to chase him off this time.

Heck. In video games there might be a “drinking the potion” animation that definitely leaves you open to attack because you can neither block, dodge, dash, nor disengage while you are drinking a potion. Many games have an animation of drawing and stowing weapons when you switch and you might definitely get merked while you are putting your longbow away and drawing your sword. Video games arguably do it better, but if I try to enforce the rules of action economy, my players get all whiney.

Start the players at 1st Level. That way you can learn the game at the same pace that they are learning their characters. Probably just run a one-session adventure at 1st Level. Then everyone levels up together. You all learn the characters new features together.

The PCs are a newly-formed party of adventurers who have just arrived in town. They all know and like each other and are excited to start their adventuring careers together. They ask around town for rumors. Every townie is concerned with one of three things:

Quest A

Quest B

Quest C

Other than these three things, there is not much else going on in town. ("Every villager you talk to seems concerned with either A, B, or C.") Your party chooses which quest to tackle first. You have had plenty of time to prepare each quest as appropriate for a 1st Level party. Each quest has a clear hook, clear goal, and clear reward. The players pick Quest B and you run that for them. When they return, Quests A, and C are still available. Level up and run a little downtime, check in with the players, "Which quest are you going to tackle next session?" Adjust that quest for 2nd Level and begin running it next session.

Expand that quest so it takes 2 sessions to get through the 2nd level quest without an opportunity for a long rest in the middle. This creates the expectation that sessions and rests do not go hand in hand. When they finish their second quest, they return to town, Level Up, Downtime, Long Rest, but the third quest is no longer available. Instead, the rumors in town indicate several other quests, some of which might take the PCs to neighboring towns or cities. Let the players choose where to go next session. Prepare a 3rd Level quest for the next session.

You'll be three or four sessions in by now. Your PCs will be level 3. You'll all be getting the hang of it. Start experimenting with style of quests and your DM style. Your going to have a lot of fun.

Put my PCs in a dungeon "outside of time." They could literally stay in there years if they wanted and no time would pass in "the real world." I told them this, then I told them, "Please do not abuse this or I will take it away." They stayed there for one long rest before returning to the "real world" fully rested and ready to kick the crap out of the adult blue dragon terrorizing the town. They might go back in the future, but so far, they are happy just to have had the "free" long rest.

"I cast bonfire."

"Fine. Whatever."

"...uh what...?"

"Dude. It wasn't funny the first time you did it. Just stop."

"Being creative using his resources," is different than "trying to get something for nothing." Trying to use cantrips like thaumaturgy and light (using cantrips expends no resources) to replicate the effects of higher-level spells (cause fear, blindness/deafness) is shenanigans I don't tolerate very well. He's a cleric. He can literally prepare blindness/deafness any time he wants and spend the 2nd Level slot.

Finally, he has some unnamed god of commerce in his character's backstory. But you are the DM. That god is an NPC at your disposal. If this player says, "My god told me I need to find this item," you can literally have the god appear to them, "My faithful servant, you have misunderstood the signs. A villain named Mace is terrorizing a neighboring kingdom and disrupting the free flow of goods and services across the realm. You must find and defeat him!"

"Is it wrong for me to want things" is an example of the logic fallacy known as the "straw man." Learn to recognize to and call it out: "Get your straw man argument out of here, bro. That's not what I'm talking about and you know it. What I'm telling you is that your choices at the table are disruptive to the game I'm running. It's becoming a problem, and I'd appreciate it if you could pull it back."

Now, of course, there is the other side.

It is not your job as the DM to prevent your players from doing stuff you don't want them to do, like stealing metal from the ironworks, trying to open treasure chests, capture monsters instead of killing them, or looking for a specific magic item. Imposing reasonable consequences later can be cool. Make sure it is fun and not punitive.

If a player decides to screw around trying to open chests obviously full of treasure while shadow monsters pour into the room, that is actually a very intense situation. Let them know the monsters are closing in, they fail to open the chest. The monsters reach them and attack! I don't understand the issue here. The tension you are building is less important than honoring the players' choices. If he decides to screw around trying to get at some treasure (which, by the way, is the original motivation in D&D), the chase scene you wanted just became combat against the shadow monsters.

If a player wants a particular magic item for their character, I'm usually going to find a way to get it to them, as long as it is appropriate for their level and tier of play. If he really wanted that mace for his character, maybe he runs into an NPC who heard a rumor about a holy warrior wielding just such a mace who went missing several years ago while investigating an ancient tomb in the swamp south of town. Or just use the rules in Xanathar's for searching for a magic item and have him roll for it. A regular player at your table expresses a desire for a magic item and you just ignore them? That's not cool.

Why not let him render the monster unconscious instead of killing it? Non-lethal damage is a thing in D&D and players have a choice whether or not to kill most monsters. If he expressed that desire and you didn't help him figure out how. You could have reminded him that, according to the rules, only a melee attack can intentionally deal non-lethal damage. Then he has an opportunity to capture the creature and create a new market for eldritch horse bits or whatever.

In sum:

You seem to have an issue dealing with player choices that don't fit your narrative. That's okay. You have a lot of committed players. They are obviously enjoying your game. I have played with several DMs who are very plot-oriented and become demotivated or even adversarial whenever we go off the rails, backtrack, or decide to look around town for a while instead of proceeding directly to the palace to talk to the quest-giver. I still have fun, but I relish being able to explore and choose my own path. I know DMs are preparing an adventure, and I am to go on the adventure they have prepared, but within that adventure there has to be room for player choices - even "bad" ones like taking the metal from an ironworks warehouse or putting the party at risk to open a treasure chest.

You have developed a passive-aggressive adversarial relationship with this player because his playstyle does not mesh well with your DM style. He makes a choice you don't like, so you don't honor it. He makes a request you are not willing to grant, so you ignore it. He expresses a desire to do a thing, you warn him off it instead of just playing out the consequences.

Nothing your player is doing seems out of bounds for a game of D&D. Looting adventure locations, capturing monsters, looking for magic weapons: this is all classic D&D adventurer behavior. You sound like a fantastic DM, and if you learn to let yourself roll with player choices even when you disagree with them or aren't prepared for them, you'll get even better.

My DM gave the big scary monster a big scary weapon that was obviously casting big scary magic effects every time it hit. I immediately asked about the disarm special attack option from the DMG and whether he would allow it. I was running an Oath of Vengeance Paladin at the time so I used vow of enmity to give myself advantage on attack rolls and succeeded on disarming the monster of his big scary magic greataxe. I then used my free object interaction to pick it up, moved a few feet to my left and dropped it into a chasm.

The point is, my DM created an interesting scenario with which my PC could interact. A magic weapon that could kick our asses, a chasm nearby. Then he was game when I wanted to do something cool with that situation. He didn't hem and haw about it. Finally, I knew the rules and meticulously played by them.

When the players bring you options that are interesting and cool, lean into that awesomeness and reward it. But players need to do their part and exercise their creativity within the rules and not try to pull some shenanigans BS.