Awesome game! I played on normal and still did terribly, but that's a skill issue.
You wrote all the songs yourself??? In a week??? That's crazy. My favorite of the band songs was probably Noot!Noot! Truly a bop. I loved how the penguins danced to the music and appeared/disappeared based on your performance (mostly disappearing for me u_u).
Since you asked where you can improve, here are some MINOR nitpicks but know that they are nitpicks:
The music at the beginning seems to restart every time the scene moves forward, which is jarring and slightly annoying.
You could also add more feedback when a note is hit (or missed), like a sound effect and/or particle effects or anything to make it feel more impactful.
A custom style to Godot's standard buttons and menu would make them fit in with your game more.
Overall very fun entry and an amazing first game! (Also gets extra points for all the characters being penguins)