AOO Button. On/Off. If On, it means that whenever an enemy tries to leave melee without Disengage, any enemy at all, you do AOO automatically. As an alternative, there's a single option. Selected Target(s). This makes it so that IF, during combat, you want to only have AOO trigger against certain enemies, you can select them so that you don't waste your AOO on potentially unimportant enemies.
So, 3 options. On, Off and Selected Target(s). Not super complicated here. Not hard for new players to figure out. Wouldn't take much to manage even IF you wanted to only trigger AOO on like, let's say, the Matriarch instead of the Phase Spiders or the babies.
Since (as you proposed) you only can switch the selected targets during the character's turn, this system already does not provide the accuracy of proper reactions. Examples with combat situations were provided in this very thread. What's to test here? Yes, the game could compile and run with this system. Would it be good? Not for me. Do I have to try it to be sure? No, I don't.
>This would DRASTICALLY cut down on the number of times Counterspell would prompt you. I like when counterspell prompts me. I like to think whether I should use it in this particular situation or save the spell slot. To feel the power to influence the combat.
Btw, taking it a bit further, why not make ACTIONS presetted? Fighter runs and attacks (preset for meelee/ranged/mages/boss enemies). Character uses thas potion when they're < 15 hp, that potion when they're < 30 hp. With right presets you don't even have to play the game!