Hardtail vs ATB?

Maybe find a bike/frame that can do it all: rigid/flat bar, rigid/drop bar, hardtail/flat bar, hardtail/drop bar. There is considerable cost in going from flat to drop, but there are some handlebars like the corner bar that are close enough to flared drop bars but let you continue to use your MTB controls.

I don't have any input on bike type, but when you say your arm doesn't bend straight, do you mean you can't straighten your elbow? This shouldn't be too much of an issue with any bike because realistically you shouldn't be holding the bars with locked out arms.

If you find yourself locking your arms completely straight its usually a sign that a bike is either too big or not set up correctly, you should be holding the bars with slightly bent arms to absorb shocks, otherwise everything will just go straight into your wrists, elbows and into your shoulders/neck creating all sorts of discomfort.

So it's less about agressive/upright and more about finding a position which you can sustain mostly with your core muscles, rather than supporting your weight through your arms.

Get another set of wheels for the fat bike to run 2.6 - 3 .0 to tyres

Get a hardtail, and get a suspension corrected rigid fork.

Sounds like you already have a fat bike for gravel. I'd get a hard tail. You can lock the forks out when you aren't using them.

I ride a Kona Unit X and love it. It provides all that is needed to rip trails, ride pavement, and do some bikepacking trips.