Interesting games turn me into mr.nitpicker. Here's my thoughts:
1. Low-poly artstyle + color pallete is really cute
2. I like the idea of a ranged phase and a melee berserker phase. The "Caw' SFX is hilarious. However, playing well means I never go into rage mode. Perhaps make rage rise automatically overtime to make rage something you plan for rather than something that happens.
3. The idea of collecting music sheets is to unlock spells is nice and so is using spells to unlock door. The spell swap mechanic should not involve clicking on the button.
4. You get to see what notes to play for a given spell after selecting it. It works but needs some getting used to. Monster Hunter World has a nice note visualization mechanic. Look into how it's war horn works.
5. Better auto aim + aiming reticle is needed.
6. It's better to stick to the fireball spell as it's very fast to type and hard to mess up. The other spells need more reasons to use them.
7. New Sound VFX particles in the title screen don't show up until the old one has faded away.
Overall, very unique game with unique visual flair. Do expand on this jam entry later on.