Thanks for the feedback. I'm aware of these issues, and am already working on fixing them. Firstly, I'll add a system for upgrading modifiers: you take three mods of the same type and merge them together getting one mod, that's twice more powerful. Sure, you're losing a mod, but you're saving a lot of space.
Or you can take three mods of same rarity and swap them for a single rarer mod.
As for overpowered guns and gun switching, that's gonna take a lot of balancing to fix. While new enemy types can also help, I can't think of an enemy that couldn't be able to be defeated by sheer power. I'll probably add some drawbacks and synergies to mods.
And shooting all guns simultaneously is not gonna happen.