Awesome mechanic and great level design. The aesthetics are pretty good. I think it could be most adhered to the theme if you give the player only one bullet for real, like for all the level. Because as it is, you can get another bullet. The idea looks pretty original and has a lot of potential for a longer version or for more game mechanics.
We also made our game in Godot: Mocho VS Mostros. It would be nice if you give it a try and give us some feedback.