Very specific day bag

You are in England?

Drop a message to Atom Packs. I am pretty sure they will make you a hybrid like that.

If you "put a plastic bag inside" instead of wanting a niche and more expensive waterproof pack, you'd open up way more options.

Sounds like time for a custom deal or MYOG!

How about this one?

https://www.aarnpacks.com/collections/aarn-packs/products/pace-magic-20

This definitely doesn't meet all your criteria but I think Chicken Tramper does somewhat custom orders. Might be worth looking into one of their day packs.

HMG Elevate 22L is pretty spot on

I basically pimped out a stock camelback 18L with a nice padded hip belt: https://www.tombihn.com/collections/accessories/products/hip-belt?variant=42595764404413

Pretty much has all the rest of what you specified above: ice axe loops, sternum strap, 2 bottle pockets (plus the camelback bladder if you want to use it), daisy chain. Not waterproof tho...

Fwiw it's easy to add a front shove type storage by attaching one of those elastic bike nets.

Check out the OMM bags

Closest I can come up with are some technical packs:

https://www.mountainhardwear.com/p/ul-20-backpack-1891001.html https://www.mountainhardwear.com/p/alpine-light-roll-top-30l-backpack-2020791.html?dwvar_2020791_color=107 https://www.mammut.com/us/en/products/2530-00340-40242/ducan-spine-28-35 https://www.mammut.com/us/en/products/2520-03832-2277/trion-28

You might need to sew on a hip belt or add some webbing to the back to perfectly meet your wishes.

The mammut Duncan spine is probably closest to what you describe.

Special shout-out to this pack which is almost exactly what you describe (water bladder instead of bottle holders), but it's currently sold out: https://freerangeequipment.com/products/raven

Ultimate direction fast pack 20 with your gear in a liner is an option

You could try the 30 or 20 L aliexpress running packs https://www.aliexpress.com/item/1005004459882161.html

Alpkit Gourdon is pretty close and cheap enough to modify?