I definitely took inspiration from your approach after realizing that installing proxmox over serial wasn't going to work - thanks for the ideas!Sounds the same as my setup on the second node, m.2 to PCI-E, into a Quadro for the Proxmox install. Then I have dual Sata SSDs for Proxmox and three WD 4TB drives for LXC/VMs. Running smoothly here for 4 days straight. So it sounds like a hardware issue, might be an idea to reset the Bios just to make sure?
This also seems like a pretty ideal use case for these boards - I've been running my two working nodes whenever I'm not worried about noise in my home office and they've been rock solid. I was worried that live migration with zfs as a backing store would be slow, but nvme as an l2arc + 10G and proxmox built-in periodic zfs replication makes it faster than vsan.