I think that the fire should get you away from holding shield down a lot. So why not make the fire spawn cooldown go down when you are holding the lever at max? Punish from just avoiding two whole mechanics. I also recommend using exponetial functions for upgrades that lower, like fire rate, so you never hit 0 or below (y=2^x). I still really liked every bit of polish put into the game, and for future jams I highly recommend giving some early builds to family or friends, and tell them minimal amount of information. And if you can, watch them play. You'll learn a lot, even from your own game!