You just nailed what you were looking for! This game is a great, which had a bit of Just Shapes & Beats feeling (but that can only be a plus). :P The music is great the graphics are fitting.
However there are some stuff that I would like different. Since the game has a rating system, I think the hidden HP isn't needed. Instead, I would use a "if you take too much damage in a certain time, you die". For example, if you took 5 hits in a second, that is a death, but if you took 2 hits and then 3 other after 10 seconds, you wouldn't die.
Secondly, the shooting feels a bit off... Shooting in different directions with the arrow keys instead of mouse feels intuitive, especially when you need to shoot accurately. So I switched to a controller, but sadly I could still only shoot in the 8 direction... Which was a bit annoying. I would recommend using 360° aim with the controller, and mouse shooting with the keyboard. Or actually, a bit of auto-aim could have fixed this issue.
Overall, this is a great game, I really enjoyed playing it; a great submission. :)