The component shortage issues aren't getting any better, but I was able to find substitute parts for the PSPi Compute, and a new prototype is on the way and will arrive within a couple weeks. The new board is a total redesign and barely resembles prototype 4 from earlier this year. More details found here:
https://othermod.com/pspi-compute/

There's also a Monoprice Ultimate interface board for the UM2+ print head in the same order. I'll give some details on that once I verify that everything works as expected.