Eh I'm not a fan of slowing things down for reactions. The game is turn based, so it should stick with turn based and not add psuedo Quick Time Events. In a turn-based system, I expect to be able to leave my computer at any point, and come back not having missed any opportunities.
I'd prefer that time froze and a pop-up window appeared asking "do you wish to use [reaction]?"
I guess if they can slow down and add a timed popup... They can easily pause this moment and freeze the game until you click that popup.
My fear with a pause is that it would be like... In Solasta. I love this game but those pause appear more than once on every turns. I can deal with it but I don't think lots of players would like this.