The PCBs arrived in the break between holidays, and I got to assemble one of each. First the Raspberry Pi Pico shield:

I tried it without the crystal first, with a bodge wire to a free pin to generate the master clock with PWM. Once I had it verified working that way, I removed the bodge and soldered the 24MHz crystal, and it works flawlessly. Then I moved on to the S2 Mini shield:

I tested the previous version of this with the crystal connected with bodge wires, and it didn't work, so I had very little hope for this, but turns out it was the fault of the bodge wires, and it works perfectly fine with the crystal soldered on the PCB.

I think that this pretty much concludes the project. I will make the design files available, and that's it.