Solasta UI is fantastic, the only issues I'm seeing people complain about on it in this thread are basically that it's (1) too big, and (2) looks "too modern". So literally make something similar so we have a button for actions, bonus actions, action spells, bonus action spells, slim it down a bit and give it a fantasy makeover. One thing that I feel Solasta handles much better than BG3 is reactions. It literally pops up and asks me if I want to spend my reaction on attacking this creature or not, while currently BG3 just forces me to either have my character automatically use their reaction on whatever enemy moves out of my area first, or toggle it off so they don't use it at all. And reaction spells aren't even cast as reaction spells because you have to cast them ahead of time, wtf is that? At least in Solasta if I wanted to cast a spell like, say, burning hands. I click the spells button, I find burning hands which is very easy as it's listed in the area for 1st level spells and has an icon that sets it apart from other fire based spells nicely, and I target where I want it to go. Plus, it shows me the area it effects in a 3D cone shape, so I can see who it will hit and can properly target enemies that are higher up as well.
Currently, if I wanted to do this in BG3, I would have to sit there clicking the arrows to scroll through my hot bar and searching through several pages of spells, abilities, potions, scrolls, etc. until I found it, then select it, and then instead of the proper cone shape it displays some weird area of effect thing on the ground in front of me which I can't really target properly and which doesn't seem able to hit enemies at higher/lower elevations. Considering how much verticality is in this game, making it so my spells can actually target people higher than me like they're supposed to would be nice.
