Generally sounds like a feature you would choose what level of smite and toggle it. That would be my guess to keep the game flowing, but who knows what Larian will do.
Later levels presuming it'll get even worse, that is if opportunity attack increase, so instead of just one OA you have multi and you'll be ask for each one.
Hellish rebuke was one of the warlock spells I was referring to earlier, you prep before hand and stays on you till it procs.
Having multiple toggles you have to switch seems a whole lot more cumbersome than a reaction window when you make a hit. Especially I can see people forgetting to set it properly before each attack.
According to 5E rules, you can only have one opportunity attack per round.
Honestly, your method of just having toggles just means we'll have lots of toggles to manage. I don't see how that's easier and more intuitive than having a reaction option window pop up when an event occurs that allows you to use a reaction.
