Thanks for the review! I had a lot of fun making the game, so I'm glad I took a first step in game development.
The overheating gun has definitely been a mixed bag for people. I think most players missed the 'overheat reduction' upgrade, which can be found before the 2nd boss. If you missed it as well, it would explain issues with later bosses. I would agree that the overheat itself is probably too long.
I think I'll add a map to suggest paths which the player can explore. Hopefully more people will find it then haha.