I have now fixed the two problems I found with the PCB, and uploaded the Gerbers and the Fritzing files. The keyboard uses the same bootloader and CircuitPython firmware as the #Fluff M0 — that's the convenience of having a board that breaks out all the pins.

I'm still working on the Python code for the mod keys, of course — I will post it separately when it's ready.

Here's the schematic of the keyboard: