I feel this way about the GBA. It's modern enough to offer things like a 32-bit pipelined CPU that can be targeted with a modern C[0] or Rust[1] toolchain, but you are still extremely close to the metal. Even decade old PCs can run emulators that focus on accuracy rather than raw performance, and the physical AGS-101 has pretty nice backlight without any mods. Plus many of the games have aged extremely well, with the graphical style of e.g. Pokemon Emerald still being closely followed in modern indie games.
I, too; am a huge fan of GBA homebrew dev, in high school; honestly, it was supremely helpful in me mastering the C language, and; even better - it led to my DS development days, which was far, far, more fun.
Dealing with the DS, especially; wow. Two screens, touch input, I gotta say, as a developer it felt like an open canvas; even if I was; like...15 at the time, lol.
It's what led me into iOS development when the App Store was officially announced. (I must've been 18 at that time? 19?) I, again; saw this as a multi-touch open canvas for creating basically anything my mind could imagine, at the time; the extreme restrictions Apple placed on App Development were actually nothing compared to the limitations of e.g. the DS, much less the GBA. (at least the DS had a wi-fi stack, lol...)
[0]: https://gbadev.org [1]: https://docs.rs/gba/latest/gba/