Solasta did the reactions perfectly well, but BG3 seems to have "automatic reactions", which means you might counterspell silly spells. I don't know how they will implement stuff like Counterspell unless they also give you an option to choose when you use it.
Just add activator to reaction section. If active, game asks player to confirm reaction. If not, reaction-spells don't trigger. For Counterspell, it can be light version of spell, reflecting only threats directly pointed at the caster.