I recognize Spell Cards when I see 'em :) It's a great idea and I love the art style.
Presentation-wise, we could do with some feedback like a preview of what the spell card is about to do. You could probably scale post-game jam by having different kinds of bullets like lasers and circles which have totally never been done in any bullet hell ever nope.
Something I noticed is that the right side of the level is almost never used. In Some Other Bullet Hells, there are Spell Cards that also move the boss. Perhaps instead of throwing a bunch of different spells at the player, a smaller selection of spells with a larger variety of mechanics would have created some gameplay complexity.
No I'm not salty I couldn't beat level 10.