No matter what, to properly reflect how 5e is played, at some point there has to be room for player choice in the moment, not in advance.
In certain ways and to certain degrees, BG3 will not be able to play like a genuine D&D session. If we aren't realistic about this as regards reactions, the end result is going to be what it currently is: automated reactions with no player input beside on or off. What ever you come up with, it cannot be too costly in time and money, nor be at odds with the flow and style of the game. A proposed solution that requires us to act on enemy turns fails the latter, and very likely fails the former. Compromise is needed. If we can set some conditions for our reactions on our turn, we can easily get more than we are set to get.
