Convoluted? lol. That's hilarious. How is this convoluted?:
Shadowheart. Only Reaction is AOO. Set by default to All Enemies. Will I ever change this? Probably not. I just leave it active all the time. IF I change it, it might be to select certain targets because I don't want it to target grunts when I'm fighting a boss... maybe.
Astarion. Only Reaction is AOO until he reaches level 5 and gets Uncanny Dodge. Then I might maybe go in to my Reactions and tweak Uncanny Dodge so it doesn't just trigger on any dumb enemy but maybe only triggers on someone's attack who deals 10ish damage or more. I open the Reactions screen. I click on a button to switch it to Damage > X, and I put in a number. Done. Now Uncanny Dodge will only trigger if an enemy does more than whatever I want it to.
Druid. Only Reaction is AOO unless I learn the Absorb Elements spell. The default for that spell is All Types, which are Acid, cold fire lightning and thunder. If I'm a tiefling and therefore fire resistant, I might click a button and switch it to Selected Types and deselect fire. Again, how is this convoluted?
Fighter. Only Reaction is AOO until they learn maneuvers or reaction spells. If maneuvers, they are, by default, not selected as Reactions. IF you want to couple one of your maneuvers to AOO, you can go into the Reaction screen, pick a maneuver and activate it as a Reaction, and now it'll trigger when your fighter AOOs unless you disable it - which you can do during combat even during your character's turn.
Paladin. Only Reaction is AOO. When you learn Smite, default is to not couple it with AOO. But, if you want it to be activated on AOO, go to the Reaction screen, activate it and select your preset option. All Enemies? Enemy Current HP > X? Whichever you like. IF you want to.
Wizard. Only Reaction is AOO until you learn a spell that is a reaction, like Shield. IF you learn the spell, the default is to activate Shield whenever an enemy hits you, but only IF the enemy will miss with the +5 AC bonus that Shield gives (and, of course, if Magic Missile targets you). This is by default, so the newby player doesn't have to do anything. But, if they DO want to do something, open the Reaction menu and switch the preset from All Enemies to Magic Missile Only OR a preset option where the user, again, can easily set a value to determine the trigger.
None of these is super complicated or convoluted. It's no harder than the original BG and IWD games where you set your character's basic combat script. If anything, it's WAY simpler and easier to understand.