I really like this game! It's really hard but also really fun!
I think that there are two things you should change though:
1. I think you should make it so that dying would automatically restart you and allow the player to pause to change the mouse sensitivity. And if you don't do that, at least save mouse sensitivity to make it easier to restart.
2. Make the enemies spawn continuously. I am not sure if you want that but I think it would be better. When there are so much variance in the amount of enemies it is hard to control the difficulty level, but if you can change it so the enemies spawn continuously and sustain a certain amount you can make it more consistent. But the variance in amount of enemies also changes up the game play and allows for more interesting situations, so I am not sure.
Anyways, it's really fun, an original take on the theme, and a great game!