Wow. I feel like I'm having to defend something that I'm just open to testing and trying if Larian wanted to do it. I'm saying it COULD be done and done fairly well, and it might even be the better option. Maybe.
I REALLY think you guys are thinking this is super complicated or that it wouldn't work to manage Reactions - without even being willing to try it or test it.
How is this hard or complicated or time consuming?
Reaction Screen 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.
That right there handles MOST reactions for MOST classes. No popups/prompts needed, and the result is 99% the same every time. Why? Because AOO doesn't trigger that often, and when it does, you usually want it to be on the first enemy that actually triggers it. Right?
And Uncanny Dodge. Again. Not complicated. Super easy. When you get it at level 5, it is set to All Enemies. So, the first enemy that attacks your Rogue would trigger it unless you say, "Hey. I don't want it to just trigger on the first enemy. I'd like it to be worth more than that. I'm only going to have it trigger if an enemy does more than X amount of damage. Again. How hard is it to switch it from All Enemies to Damage > X? And when would you want Uncanny Dodge to trigger other than one of these two? Maybe selected targets? Again... I was suggesting the option to even be able to do that. It's not like it'd take players whole minutes to set this. We're talking very easy, minor setups.
And Shield. When would you want Shield to be cast? If an enemy hits you with a weapon by 5 or less or they cast Magic Missile. Again, easily preset so you don't have to worry about prompts every time an enemy hits you or casts Magic Missile. When would you cast Shield? Enemy 1 runs up and attacks you and hits by 4. Wouldn't you cast Shield to block it? Most likely. So why not automate it? Don't want Shield to be used as your Reaction? Click. Reactions pull up. Click. It's turned off. Want it to only cast on Magic Missile? Click. Reactions pull up. Click. Magic Missile Only preset. Want it to cast if the enemy's potential damage is more than 10? Click. Reactions pull up. Click. Max Damage Potential > X. Enter 10. Done.
But again, there are defaults that can be set so that new players don't necessarily need to know all the nuances of the preset system. Even a Counterspell spell list would handle most scenarios where players would want to cast a spell.
Note: I tried a blanket preset idea for Counterspell, but it doesn't work with blanket presets (such as spell level and above). You can't just set Counterspell to trigger against all 3rd level spells or higher because someone could cast something like Beacon of Hope, and it would trigger your Counterspell, knocking out your Reaction so you couldn't cast it against something like Fireball or Lightning Bolt. But, again, that's why the Default Spell List would exist. On the Spell List, there would be the most common spells that players would want to use Counterspell for. So, you'd have spells like Fireball and Lightning Bolt on the list, but you wouldn't have spells like Misty Step or Barkskin or Beacon of Hope or Healing Word checkmarked.
I was even thinking that a blending of the prompt and presets would be perhaps the best approach. You have presets that cut out a good portion of the prompts, but then if a preset is triggered, a prompt appears asking you to confirm. Example: Counterspell is set to trigger on Darkness, Fireball, Lightning Bolt, Dominate Person... Enemy casts Darkness. Prompt. Let's say you reject. Enemy 2 casts Barkskin. Doesn't trigger Counterspell Prompt because it's not on the list. Enemy 3 casts Misty Step. Doesn't trigger prompt because not on list. Enemy 4 casts Fireball. Prompt. You accept.
This would DRASTICALLY cut down on the number of times Counterspell would prompt you.
I'm just saying. It COULD work, and it COULD work fairly well for most Reaction - I would think, at least. I'd at least be willing to test it.