Very fun game! I killed the boss :D
I think there are a few things that could be looked at. When I was finished with farming and maxed out my ship, there's no need to collect gems anymore (which is fine). So it's just surviving until the boss spawns. If you fly constantly in a single direction (e.g. just go left), I don't think it's possible to die, because enemy bullets despawn when they are killed and your wall of bullets will kill everything before they get close.
The farming minigame was interesting but at some point the income was enough to just wait a few minutes for maxing out all ship upgrades. But the game pauses when not in view :') so I had to keep it visible while I was afk farming.
Good stuff, though :D