This is the test fixture I had in mind when I started this project:

It mostly works... I made all the interconnections with individual wires. Gross. The next iteration will be a proper PCBA with pogo pin receptacles, and nice reliable traces running from the DUT to the Core Test Matrix. The biggest challenge is the extra resistance in the wires to the Core Test Matrix, making that part of the system unreliable. Everything else works great. I'm very happy with the mechanical moving bits:

Here's some of that nasty wiring:

Did I mention wire harnesses are a time suck?

A lot of my learning and inspiration came from these guys: https://fixturfab.com/

This video highlights the challenges in designing these things... 

...and justifies why it's worth paying them for the service!

Nonetheless, my fixture did help me get through testing all the boards, so now I can move on to kitting everything for the workshop!

Oh, and finish the firmware...