My man how you make a shooter without SCREENSHAKE?? Thats like the most essential thing for game feel, come on.
Sound is nice i guess, no music. I guess i could recommend "bosca ceoil" (here: https://boscaceoil.net/) if you want to do it completely solo, its free and not too complex yet still allows to make some listenable tracks, though still have to know the basics of it so id suggest to look up a tutorial for it, Brackeys made a good one.
Also dont think i felt any freeze frames (you know when a game like stops for a few miliseconds to add more impact to the action) which is also a minus cuz once again, thats like one of the essential and super easy ways to add GAME FEEL.
Id also suggest to make the friends at least try to avoid danger, even if not like super advanced, cuz like it feels they just walk directly into traps and enemies which is dumb.
Possibly adding some way to move faster could be nice as sometimes it feels like im juuust a bit too slow, maybe like move faster when not firing? Could work imo.
A small neat change would also be to display the controls at the start of the first level so people like me who didnt check the manual dont get too confused.
AAaaand thats all i could think of i guess. Good luck or somethin i dunno.