Magic spells stored on cards. What? What else would it mean?
Took some inspiration from Balatro here, not really in terms of gameplay, but moreso in the way that you modify cards by sticking things to them or otherwise changing them. Making the spells into cards keeps them organized in a way I think makes intuitive sense, where each “weapon” you have is represented by one card containing all its information. Plus, cards are already very good candidates for receiving magical properties, thematically speaking. In other words, cards are just cool.
… And also, for the kind of game where you, the player, are the bullet hell, it’s kinda perfect for your weapons to be “spell cards”, isn’t it?