All the components slowly arrived, and now I only have one thing missing: the diodes for de-ghosting the keys. Unfortunately, this design uses through-hole diodes, and I only have SMD ones in my drawers, because I avoid through-hole parts whenever I can. So I had to order them, and they take a long time. I guess in the worst case I could solder some wires to the SMD diodes, but I don't really feel like doing that for 36 diodes.
While waiting for the diodes, I should probably start looking into programming the pro micros with the right firmware.