Now that I have the project tested and working, I added the gerbers and Fritzing design to the files, so that other can also build it. It uses the bootloader and CircuitPython firmware for #Fluff M0, and I added an example with the original Atreus layout to the ukeeb library: https://github.com/deshipu/ukeeb/blob/main/examples/flatreus.py

Here's that layout, taken from the Atreus kickstarter page. Of course you can easily modify it for your own tastes.