Aww, thank you for checking my game, I really appreciate the feedback. It is clear to me that my game needs some refinement, so I'm planning to work on an update once the voting period ends. That could included additional bosses, and your twin boss idea could work nicely with my other friend's suggestion of a boss fusion. I'd also wanna explore bosses where the screen keeps scrolling forward, so you'd need to be constantly moving.
Oh yeah, I did make my music, I did struggle a bit. But I think a mix of chiptune and some synths/midi instruments made the music sound game-like enough with some energy to it.