ok how about this:
When toggling reactions player can exclude enemies. "Yes, I want to perform attack of opportunity but not against THAT enemy. Maybe also not against that one". With a clear action cue, and presumably smaller engagements this should be enough to have a pretty good control over to whom characters will "react to" and with what.
Exclusion would be per-reaction. So you could not include one enemy in "attack of opportunity" but have his tagged with something else.
The problem with this is that you don't necessarily know if you want to react to a specific enemy or not until you see what the enemies do. I may intend to save my reaction to shield against that barbarian that is low on initiative order, but if I see the spellcaster casting some huge damage spell instead of the cantrip I expected he would, I might change my mind and try to shield or counterspell the caster instead, and my choice of which to do might depend on whether the spell the caster is casting is save-based or attack-based. So, combat flow has to be able to stop at any given moment to allow players to make an informed decision on the fly.
If it was predetermined reactions, players have to be able to set multiple different possible reactions that they could take per enemy for it to in any way reflect actual 5e mechanics for reactions, and if you allow setting multiple possible reactions per enemy, you then have to pause combat to give the players an opportunity to decide which one they want to do. No matter what, to properly reflect how 5e is played, at some point there has to be room for player choice in the moment, not in advance.
