Yeah, I too was missing the music sadly in a guitar game.
I kept dying even on easy mode, so maybe make easy mode a little....easy
The movement felt great. The aiming, the rotation of the gun felt a bit slow, so it didn't feel as good to control, it felt it was lagging behind my mouse.
I would like the health and power bar to be in the same spot (top left probably) as it's hard to look at both at the same time. Also I was not really noticing when I was about to die in the heart bar, it's obvious if you look at it, but we don't look at meters during action, so the "bloody red vision when you're about to die" while tropey maybe wouldn't hurt. Or maybe a dark souls health bar which is easier to see than the more compact version because it's longer.
I liked the legendary and rare drops choices, however, there weren't enough options at least in my 3-4 runs to make them strategically different. Obviously you didn't have a lot of time, but think of that in your design, a choice should end up affecting the gameplay, like in vampire survivors, where it seems to be inspired by, each weapon and powerup does completely different things.
It wasn't clear what the batteries did until I got my first power up. Maybe making the first power up come a little earlier (maybe as soon as the first battery) would help, as well as some labeling or iconography around the blue power bar. I don't think I even noticed the blue bar was filling up for a couple of minutes.
I liked the sound effects for blowing up enemies, I think I was missing something for shooting the gun also.
It's a fun game for a game jam, but I think it would need some sort of original mechanic or sauce to make it work as a published title. But if you wanted, the basics are pretty solid here and you could turn it into a larger project if you wanted.
Great job making the procedural levels, they seemed to work well and that must have been really hard to pull off.
Good job on such a short time frame!