Thanks, really interesting! You've really carefully considered this while making the game. I like that you actually constructed the boss out of background tiles. Pretty common trick in the GB/NES days. A devlog would probably be interesting at some point!
I don't think I'm going to scrutinize the whole game so I can complain if I find something dubious, haha, but it's always interesting with new Game Boy-style games. Looking forward to the release! If you're not burnt out on the aesthetic you should consider joining the GBJAM :)
As for learning Game Boy assembly (you can make Game Boy games in C too, actually, although it's not ideal), here are some resources!