@cafou

there is a dev kit called devkitpro that uses a library called libgba. you can program in C. My first experiment though was using Zig, which worked but I had a couple issues that took too much time to fix.

I have a cartridge that lets you play bootleg roms on your system. so I just compile my code into a .gba rom file and put it on an SD card, and the cartridge lets me pick it and play it.

I used to have an actual EEPROM cartridge but it's long gone and this works fine for testing on real hardware.