It’s a nice idea and very similar, if I’m not mistaken, to the way that Braid plays: a very clever platformer where time only moves when you move, even backwards.
I’d say that this is something that TLF needed to start with rather than something to attempt to move it towards, or even add as a third method of combat. The TBS combat in TLF is fine as it is IMO. (TLA overload notwithstanding.) I’m also of the opinion that this should not be used in a shmup because those games are about pattern recognition and reaction time, and any form of time-stop would undo quite a lot of what makes a shmup a shmup.
Save the idea for another, more suitable game.