Upgrades should always improve the ship! (Burst is a bit broken)

When you say continue grinding, you are referring to endless mode? I guess that could be a problem... I'm not generally in favor of arbitrary caps, but maybe just say the each item can only receive a max of 6 (just a first guess) upgrades? That prevents the player from ever getting locked out of their options, and also curbs the weapons power in endless mode. This way the player can still experiment with strange upgrade combos, but they ultimately do have to make choices.