Awesome idea!
I was surprised to see cool-looking credits and menus with animations, so I expected the game to be well-polished. However, the game significantly lacks polish in more important areas.
When it comes to polishing — think about what parts of the game really need polishing. In this case, it's the main mechanics: movement, shooting, choosing your weapon, and balancing. All feel unpolished.
If you take the bullet upgrade, you don't need to switch your weapon. I've beat about six waves like this until it became too boring and repetitive. Probably not a good idea to make an upgrade that makes the main mechanic of the game unnecessary.
It would be nice to have a goal in the game, something for the player to look forward to, like an ending. It's easy for game jam games to overstay its welcome.