I really like the concept and execution. I had the same vision when setting out to start my game of a unifying treasure mechanic where it touches on many aspects of play, but you pulled it off better. A couple criticisms though. The fact that a bullet waits for a timer before firing rather than firing and then waiting on a timer made the game feel a bit clunky and imprecise. Also your scaling system seems off, magnets barely seem to have an effect even after several upgrades, and all player upgrades are strictly linear while the enemies seem to scale in a multiplicative way, leading you to drain more treasure on attacking enemies, even when you buy attack upgrades every level.
EDIT: upon continuing to play i discovered the ammo cost feature. I still maintain the opinion about the fire rate, the magnet is better when you have a very high value, but it's still weird, and the game is (understandably because game jam) not properly optimized