Yea it's a great project and i'm sure there are many people interested in that. For my part I found a pi boy in my country and bought it. It's a gameboy raspberri with a real raspi, i mean, not a pi zero. I bought that because it has usb port on it but it's expensive and i don't know if it will arrive because my order is stuck in same place since 28th december with no news...
You have also pi43 which seems good, cheap, but is seems to be unconfortable and too big...
You have m8 tracker which will be released i think in the next months (but i don't know)
You have nerd seq portable which will be released this year too.
But keep working on your project! I can be interested too, and with midi/minijack port it's awesome! (if it works directly with catskull electronics midi/minijack stuff (gen mdm, ym2342, gba midi synth...) it's even better! I mean using only one mini jack. It's really frustrating when you have 2 devices with minijackMidi port but which needs minijackmidi + midicable + midiminijack to work...
If my pi boy arrive some day i will say if it works good. I plane to create a chiptune machine for that, select lgpt at startup or other trackers or music programms on emulator. If you want to boot directly on piggy and poweroff when quit i can help i did that on my raspberry pi (normal one, not the pi boy)
EDIT : also, you can consider adding gate in and gate out like on M8 tracker which will make possibility to sync with korg series and with lsdj + nanoloop (many other hardware machines works with that too like akai rhythm wolf).