The popups really aren't that bad in Solasta. Like I said, paladin smite popup is annoying because it's every time you hit, which is almost every attack, and when the paladin gets 2 attacks... Ugh.
But things like Counter spell work pretty well. Enemy is casting a Fireball. Popup. Do you want to use Counter spell? Click Yes. Shows you cast a spell to counter enemy spell. Doesn't happen that often and it's super quick. Same with Shield.
And some Reactions are simpler and could be handled via Scripted Reactions. What I mean is how Larian kinda has AOO enabled currently. If enabled, and an enemy meets trigger requirements, the Reaction triggers. This could easily be done with Shield. When you enable Shield spell as a Reaction, you get to pick "Any Attack" or "Magic Missile Only" or both. Now, no popup for that.
Smite, which isn't a reaction, could be handled mostly like how BG3 has implemented the Battlemaster combat maneuvers. The only time a pop-up would then appear is when the paladin does an AOO. Then it would ask if they want to use Smite when hitting as an AOO. Again, wouldn't be too often.
+1 Smite is, imo, the ability that least needs a pop-up and can best be translated into a toggle (perferably) or hotbar icon. With a toggle that refunds your slot on a missed attack, the only difference between it and RAW is that you can no longer crit-fish smites.
So if you have Paladin Smites is implemented as such, you'll have *many* less pop-ups. If you also you don't have a character with Protection Fighting Style or have it be a toggle/hotbar-activated stance, and (I'll even concede this) you leave AOOs as a toggle, then having ALL other reactions be pop-ups would be a miniscule percentage of your time spent playing. And well worth the flexibility, especially for powerful abilities like Counterspell that you really don't want to waste on an enemy cantrip.