I get that the mechanics are unexplained on purpose, but... well, yeah, they're still unexplained. Not that this'd be the first jam game to trip over its own feet trying to fulfill a wildcard, mind you. I did get the feeling my stats were going up, but its not like I had time to try and figure out which incremental improvement is related to which abstract shape while flying in circles. I think if you wanted to expand on the intentionally mysterious mechanics thing, the trick would be to not throw all of them at the player at once?
The music shifting was a pretty good idea, a lot of these kinds of games I tend to struggle keeping track of my health.
A small note: being able to fly behind the powerup bar (and die down there) while barely being able to see through it wasn't great.
Notes aside, it's a decently fun shmup. Good job, especially since this seems to be your first jam?