Button like Passive mode enables Smite for Reactions. Enemy triggers AOO. If enemy has 1-15 HP, no Smite used on AOO. If 16-30, Smite level 1 is used automatically because button is enabled. If 31-45, Smite 2 auto used. 46-60 is Smite 3, and so on.
Now, no popups for Smite. Normal Smite attacks are like Battlemaster maneuvers, and Reaction Smites are like the above. Maybe they could even make it so players control the HP intervals. Instead of every 15 points, every 10, or something like that.