Thank you for the very detailed feedback! I greatly appreciate both your kind words and your constructive criticism, and am already at work fixing the issues you raised. I believe I may have fixed the issue you found concerning the boss fight.
Your notes about the tutorial are, to be frank, somewhat upsetting (I like the characterisation ðŸ˜) but I'm totally taking your words on board, because it's worth considering whether there's too much "yap" in there.
You're right about the upgrades, and a suite of icons is in the works to go along with item descriptions. As you can probably imagine, it's quite a lot of icons to work through haha!
To answer your curious wondering - yes, there are in fact two more levels after the Asteroid Field! The bug with the boss fight seems to have prevented you from advancing further. I'll be uploading the fix to this shortly.
Lock boxes are opened by shooting and destroying all 4 of the red lock markers on the side of the lock box. Destroying the lock box also destroys the item, so you have to be careful!
Turret mode doesn't disable your ability to move, it only locks your ship's rotation. This is to allow you to shoot to the side and behind you, without having to turn your ship around. Yes, this is risky, but there are edge cases where this is preferable to a full turn about.
I'm happy to hear you generally enjoy the game, and especially the visuals! True, explosions and other effects could use some more work. This sort of polish will have its place in the future of development 🙂