I have a Minisforum U820 w/64GB RAM running Proxmox and it runs great, but I haven’t pushed it super hard.
However, I wish I had purchased the TH50 instead because it has two 2.5Gbps Ethernet ports and mine has two but one is 1Gbps and the other is 2.45Gbps so I can’t bond the ports.
That said, I personally wouldn’t get the UM690 because it has only one 2.5 Gbps port. But then that might not be important to you. If not, the UM690 should work really well for Proxmox.