Thank you again for your kind words, guys
I'll probably focus on Kaeru no Tame ni Kane wa Naru DX from now on.
@toruzz Yes, that screen.
I can confirm that it now works fine with goomba color! Except for the screen that says "This romhack can only be played on a Gameboy color." (With goomba color set to GB Mode, it looks like a glitchy mess). Thanks for the update!
Edit: I just noticed that said issue about the GBC only screen is more than just a graphical bug. The game can actually be played with scrambled graphics on goomba color if the emulator is in GB mode instead of correctly showing the proper screen.
Sorry, I totally missed that post!
This is basically what you get from forcing any GBC game into GB mode (e.g. https://www.youtube.com/watch?v=Br9KYkSZA3E
), and that's why the "not compatible" screen is there
I use the $FF47 register to detect if the hardware is GB/SGB or GBC (which admittedly isn't the best way), and apparently Goomba Color doesn't emulate it properly. But honestly, I don't think it's worth making up for this on the hack.