I really like that you used different contexts to make the single-button requirement more interesting! I've already told you how much I like this game on discord, so I'll go right to criticisms.
I think the sound mixing could definitely use some work, it's a bit loud, and some of the sound effects that are generic boops can probably get replaced by 8 bit gunshots (like the one you use for shooting) or other, more thematically appropriate sound effects. Speaking of that sound effect, it seems like it only plays around 1/3-1/2 of the time it should, other times the cue is missed and it's completely silent (other than the background music).
I like the detail you added to the hat flying off, that's really fun. For something more nitpicky, the blood particles go over the bullet-bar-thing, so if you're gonna polish this up definitely fix that. You also can't change your difficulty after you lose.
Speaking of the difficulty thing, I don't like that it's the only thing that uses the mouse. I know that it seems like a dumb comment, but part of the charm of the game is that it does so much with just the spacebar. I'm always a big fan of mechanics bleeding over into menus, so I think a really cool way you could teach the player about your game is to use a little bar similar to the bullet-bar to let them choose their difficulty at the start. Like press it once and then press it again to choose the difficulty. Maybe that makes choosing the difficulty too annoying, but I think it's an idea at least worth trying out.