Yeah, I think I was thinking about the keys/buttons more than, and there being clear that you are might want to make an action... I agree that the camera-work part of QT events sounds horrible here! Would have to be a bit more low key, but with the reaction-box making enough movement to draw attention from a corner or something.
Your idea with pausing sound like it can work! Could be a hotkey for "pause between next initiative order" (I guess with swarm AI counting as a single one). Then a marker is shown that it will pause, while finishing current animation.
Point is, there are lots of options here. One of the things I like about 5e is counterspelling and shielding, and the tactics of picking who-what-when.
Ya, I think it's a compromise that might be able to accomplish most of what players want while not overhauling the Reactions system.
