Well, if you would like to go mini (with ECC and 2.5G) you could take a look at this one:
https://www.aliexpress.com/item/1005006369887180.html
Not totally upgradable, but at least pretty low cost and modern with an optional SATA + NVMe combination for Proxmox. Shovel in an enterprise SATA and a consumer 8TB WD SN850x and this should work pretty good. Even Optane is supported.
IPMI could be replaced with NanoKVM or JetKVM...
That looks pretty slick with a standard hsf for the CPU, thanks for sharing
Nice indeed. With only 2 nvme slots, what drive configuration do you have in your mind? Backup from nvme to HDD locally and another device remote?