Thank you for the review and congrats on getting to the end despite the many shortcomings of the game! The final boss is not intended to be beaten, but is indeed beatable, with the only change being one small line of dialogue in the ending. It is there solely as an option for those who want a truly hellish challenge for whatever reason. :)
(I will be seriously impressed if someone manages to actually beat all 6 bosses within the voting period.)
The only real "Metroidvania" aspect is being able to backtrack with the pulse rifle to beat whichever "second" boss you might have skipped (either Rust or Construct), as well as climbing all the way back to the beginning and fighting one last (thoroughly unfair) secret boss.
I 100% agree on the stuff that should have been added, especially some music would definitely have made a huge difference for the game. Sadly it was not meant to be, as coding the boss behaviors took up basically the entire month. It was one of my goals to truly test the limit of what I could make in a month, as well as how far I could push the boss difficulty while still being able to barely beat them as the developer - and I guess I found out in both regards!
(For the record, I think both the final boss and the "secret" sixth boss took me upwards of 2 hours each to beat.)
With that said, I might try to finish this game after the gamejam, since I'm pretty happy with what I was able to make!