Nicely done! I like the choice the player is given as to how quickly they want to up the difficulty (stay behind and farm XP to level up, or move quicker). I see there's a timer that is counting down - I assume this is the amount of time you have to finish the game? A original and cool way of making the progression work. I also quite liked the various upgrades, good work.
As others have said though, the movement system is...janky :D Having to basically fly away, then slowly turn around (yes I know it can be upgraded), then shoot for a bit while slooowly backing away, and then run away if too many enemies get in your face isss....eh. It felt very clumsy!
I didn't get that far, about 3 bosses in or so, but the bosses were by far the easiest. All you had to do was park with the boss just inside your turret range, and then just sit there. Their random attacks had a very small % chance of hitting you, so you really didn't need to do anything else. I would suggest maybe adding some kind of slow seeking attacks to them to force the player to move around (although with the current moving system that would probably mainly be frustrating).
Still, very polished, very nice, good sounds, good music, good progression - great job, all in all!