Thank you for the feedback!
For the upgrades part: there are 3 “unique” upgrades (would like more) and several small upgrades, such as if your low health, you do more damage (changes your bullet color too btw), health, max health, etc.
As for the enemies, the further you progress in the waves, the more enemy types are introduced. There's a bounce enemy, an enemy that splits into 2 other enemies when dead, and 2 types of bosses. But yeah, 1 or 2 more types would have been cool, although balancing would have been difficult with the first waves.
On the visual part: I agree, some direction would have been better :)
For theme: 2 of those upgrades are tied to and affected by the music. Every time you lose a certain percentage of your health, an instrument will fade out of the current music. For example, if you take damage and the bass fades out, 1 of the upgrades will no longer work, encouraging the player to heal and play more aggressively. Also, you can shoot with the right mouse button instead of the left, which also depletes health (and thus fades out the music instruments, which in turn makes the unique upgrades less effective).
Again, thanks for playing!