Really dope concept. Just some feedback:
-(Some of these are nit pickey and completely based on what you're going for with the game)
*also i didn't play through the whole game so excuse me if something changed later on
-I would say that have a closed door sprite for the exit could be good so you can see where you need to go at a glance of the level
-I would say that you should add an empty ammo noise when you try to shoot as well as making it more obvious that you have limited ammo when you first introduced it because it took me a couple seconds to realize there was ammo considering the limited feedback and the limited tutorial, yes its in the corner I just didn't immediately look for it there
Now for some ideas if you would like some:
-I think you could have a lot more freedom for level design if you added vertical shooting
-Also I think screen wrapping--for the player and the bullet--could perfectly suit this game, not in all levels obviously but still
Thanks for reading