It’s not the latest generation, but the Supermicro X10SDV-4C-TLN2F can be had on eBay for around US$250 shipped from China, including a Xeon-D CPU. See:
https://www.supermicro.com/en/products/motherboard/X10SDV-4C-TLN2F
and
Dual 10 GbE, m.2 slot, up to 128 GB ECC RDIMMs. Only one PCIe slot, so expansion is limited, but with only four spinners anyway it sounds like a pretty good option. I bought one after I saw that thread, and I don’t even know what I’d use it for yet.