I like the gameplay here.
I only wish it had music. If I was doing it, I would have something tense and driving, that got more muffled, slightly quieter/ distorted when you hold down the shift key. Create a track, and then run it through some distortion (specifically a low pass filter and maybe some extra reverb) and add in a heartbeat sound so now you have two tracks to fade between when shift is pressed.
on a related note, I just recently listened to the Dunkirk soundtrack.
Let me know if you want to collaborate, or if you are happy with the game as-is