I definitely think giving the players control over reactions would be far better gameplay than any toggles system, no matter how well designed the toggles are. I am unconvinced by the argument that it would slow down gameplay in multiplayer, because combat is already turn-based and players can take longer on their turns. A simple timer of say 6 real-time seconds on whether or not to use the reaction would largely remove the complaint about it being an extra pause in the game, and the game is already turn-based so synchronizing the slowdown as a character chooses to take a reaction could probably be done as well.
It's true that characters could have many possible reactions, but they're triggered in different ways, so you wouldn't see one enemy action get ALL the reactions to pop up. Characters get one reaction only a round, so if they use it on anything, they'll have no more triggers. ... Actually, the toggle system could still sort-of be in the game as the action your character would take by default if the timer runs out on a reaction.
At the very least, I want to see "Ready Action: Attack if enemy closes", because right now if warriors don't have quite enough movement to reach an enemy, they're stuck. It's really unsatisfying watching enemies stroll up and hit your character because you don't have any ready action to attack.