And the best way of doing this is to set a toggle, so that the sneak attack procs on the first applicable instance automatically, but you can turn the toggle off if you don't want to waste sneak attack on a low-hp enemy.
Eh, I still think Solasta got it right with pop ups for that kind of stuff, functionality wise.
I agree with you for many abilities in 5e, like Smite. But 90+% of the time you'll want to sneak attack on the first attack you make during a turn, especially in BG3 where non-offhand-attack bonus actions are more prevalent. So in this specific case a toggle will result in vastly fewer clicks than a popup. And there's never* a situation where you won't use your sneak attack on a successful AoO, so a popup is useless there. And as @Niara says above, a toggle that automatically turns on after an attack is even more convenient.
*Unless you're forced to make an attack against an ally, but aren't forced to use sneak attack on said attack. Somehow.
