Anyway, I respectfully disagree with your reductionist statement from above about the reaction mechanics, both in D&D and in BG3 and, even if they do not include the usual rules of the core extended rulebooks; the reactions are a core mechanic of D&D5e that in my experience is used extensively so IMHO any resource expended to make it work smoothly and following the 5e rules would be a time well spent.
Agreed. You've identified 15 different reactions, so it seems a strong possibility that the current auto-activate system will not work well; particularly when there are many combatants.
It does not seem to me that there is an overhaul needed. If you can handle reactions which auto-activate, you can handle reactions through player agency. The main work needed would be UI for activation and preference setting.
