For the reaction system, it seems to me, that

1) Having a passive reaction system like we have now is unpopular, because it is too inflexible. IE you can not choose to only block attacks with the shield spell from strong enemies. The goblin attacks you, you cast shield, your reaction is gone, and the Minotaur attacks. This can be solved with some homebrew (like cutting word now) but that creates its own problems

2) Having an active reaction system like Solasta will slow down the game quite a lot. After EVERY attack of a Paladin, the game will ask you if you want to smite, bards have reaction for attack rolls, saving throws, skill checks, basically after EVERY dice roll in the game if you play with a bard, you will have to click away a pop up menu. This allows a high degree of flexibility, but slows down combat and is annoying

My solution:
Bring back slight real time with pause elements (might even make some BG1/2 purists happy). One key (lets say the space bar) is assigned as the "reaction" key.
During any animation, if you click the button, the game stops and a pop up menu with all possible reactions comes up. That means you will not have to select no for smites after every attack, but can choose yourself when you want it, without loosing flexibility as described in 1)

You could then also assign automatic reactions as they are now, with a toggle, if you want to. So if shield is toggled on, every time my wizard is hit, they cast the shield spell.
If shield is not toggled on, if I am attacked, nothing happens, unless I press space during the animation, and then select the shield spell as my reaction (if more than one reactions are available, for example a bards cutting word etc)

This keeps the advantages of both systems, and allows for flexible reactions without homebrew, as they are in the PHB, without making this a pure tactic simulator with hour long combats thanks to pop up menus every three seconds.

- introduce a reaction key
- individual reactions can be toggled on, if shield is toggled on, every time my wizard is attacked, they cast shield
- individual reactions can be toggled of, then, only if I press the reaction key during the appropriate animation a pop up menu appears and I can select what I want to do
- keeps the flexibility of the Solasta system without sacrificing the simplicity of the current BG3 reaction system