Wow It is really cool to see you on hackernews as well. I have programmed 2 different gameboy advance games for game jams in the past. But building it from scratch is really hard. Most resources are down with the exception of TONC https://www.coranac.com/tonc/text/ So you will have to use the spec sheets a lot. Gbstudio is very well made and easy to use. But if you are looking for a challenge i will leave the trivial excessive to you, the reader.