I did a little bit of GBA homebrew development back in its heyday. It is a really fun little bit of hardware to hack on.

It's powerful enough that you can write your whole game in C++, but not so powerful that you should rely on the system allocator for anything.

It's in a lovely sweet spot for a particular dialect of zero-overhead C++ that's really fun to write.

As a simple example, I had a little class library of data types that were crafted to exactly map to the hardware registers of the GBA. The whole thing inlined away to nothing.