I got to give the wonderful folks over at https://serverfactory.com/ a *giant* shout out and thank you: they have been nothing but courteous when I reached out finally in December about the seemingly DOA (dead-on-arrival) motherboard more than a year after the original purchase. They had me return the board and arranged the rest with ASRock, the replacement arrived last week, but it took a bunch to get the minimal hardware to test it (because I didn't want to disassemble/impair my running home server).
Currently happily chugging along with a cheap 4c/8t i3-12100F and a 32GB DDR5 ECC RAM, going to move it to the workbench and hook it up to my home network over the weekend and slowly start testing it further.
Mechanically there are plenty of challenges if I decide to go through the original plan (put it into the fully passive Monsterlabo case with a powerful GPU), but given that the main server is happy, I have all the time to experiment.