Here is the first prototype that I consider as base going forward for a bigger test setup.

That's how the PCB arrived:

And I'm very happy how they turned out (baked in an old toaster):

For the prototype: All 8 displays can be updated individually, and purely to test the system the first 4 displays (from the bottom!) and their CS lines are controlled by the first shift register (even though it could handle 8) and the second row is controlled by another shift register daisy-chained to the first.

This setup already runs QMK, you can find the firmware on my git repo: https://github.com/thpoll83/qmk_firmware/tree/PolyKeyboard/keyboards/handwired/polykb

It's not actively maintained as I moved on to another keyboard layout (and MCU) but I wanted to keep it for reference.

Thanks to QMK it already perfectly works as keyboard and I can use it's integrated layer system I can update the keys whenever a layer switch occurs.

I purchased a cheap logic analyzer to investigate all SPI and shift register signals as troubleshooting only on the software side is close to impossible (at least for me).

Here a video of the working version:

https://twitter.com/thpoll2/