You can call it a "prompt" rather than popup if it pleases you all but the result is mostly the same.
We'll just have to manually react this prompt each time to pause the game and read "a popup" (called micro action bar in The Composer's feedback doc) just to understand what every prompt is about...
Yeah. That's why I think most Reactions could be handled via presets. I mean, would it hurt a player's gameplay strategy that much if they set Uncanny Dodge to something like "Only trigger if enemies deal 10 or more damage" as opposed to saying No on several prompts because enemies attacked and did only 1-9 damage and you are saving it for an enemy that might do 15?
And Shield. How often will you NOT cast it if an enemy comes up and attacks who can deal 10 or more damage, or if they are casting Magic Missile? And if there are situations where you wouldn't, why not turn it off during your turn or tweak the preset so that it triggers only on Magic Missile or whatever?
Either way, the point is that many of the reactions really could have some basic presets to at least cut down on a lot of the need for prompts. You could cut down the number of prompt needed reactions to a select few to keep combat flowing fairly smoothly.
And the more I think about it, the more I'm thinking it really wouldn't take that long to manage presets.
Astarion would maybe have AOO and then maybe Sneak Attack Reaction until level 6 when he'd gain Uncanny Dodge.
Lae'zel would have AOO until maybe maneuvers reactions. So maybe 2 you'd have to set for her?
Shadowheart would have AOO. That's pretty much it.
Wyll and Gale would maybe have more depending on spell picks, but how many reaction spells are you REALLY going to pick and prepare every long rest. And for them, you'd probably turn off AOO presets if you were going to have them focus on spell reactions anyway.
A paladin would have Smite reactions, but that could be preset easily.
A bard would be the potentially biggest and most difficult, but it still could be done with most bard reactions.
Anyway. I'd at least like to test it to see just how well it could be done. If Larian at least implemented a preset system and let us test it, just to see if it works, I'd be willing to try it