A while ago I was a little into controller hardware hacking and did almost the same thing but for an MSX! Even wrote a blog entry, but unfortunately in Japanese (with pictures though): https://blog.qiqitori.com/2022/09/%E6%9D%90%E6%96%99%E8%B2%B...

And one year later I also used a Raspberry Pi Pico to convert a USB controller's signals to MSX: https://blog.qiqitori.com/2023/09/using-a-usb-hid-game-contr...

(And later, USB controller to Nintendo Switch, https://blog.qiqitori.com/2023/09/playing-on-the-nintendo-sw...)