Oh boy, I love rhythm games, and this one is super fun! The timing window is fair, and I really like how the music continues and resets seamlessly even if you fail at any point. I do feel like the visual cue from the background somewhat takes away from the overall game, because it distracts from your beautiful animation in the foreground.
It's probably much harder to do, but if instead the cue can come from your boss animation, or somehow from the music it self (similar to rhythm heaven or melatonin), it would add a lot to the gameplay!
I also really like your transition screen, gonna have to steal that for my own games xD!
Overall I really loved the game. Amazing work!