I added joycons to a gameboy advanced sp if you actually build this let me know. software component is written in java (SIGH I KNOW) you will need to get java up and running on a pi zero, its a pain in the butt. oh and for some reason the joycons will disconnect if wifi on the pi is enabled, youll want to run the pi headless over usb (google it)
also you can just order from pcbway to get the board https://www.pcbway.com/project/shareproject/ground_boi_signal.html
heres a huge iso you can download, burn it with rufus/balena/ idk a tuna sandwich https://drive.google.com/file/d/150eEt6jt2sRTaDK3TDJbanNfYEL_4Ezm/view?usp=sharing
these instructions are intentionally vauge because you should not build this.