Not saying that this is the best option, but there is an Asus WS C422 DC mobo, which is sometimes available for cheap.
- LGA2066 mobo with 8x DDR4 Reg. ECC slots.
- 6x SATA ports; 1x SFF-8643 (U.2 only); 1x M.2.
- 3x PCIe16. With declared x4x4x4x4 bifurcation support. 1x PCIe4. All are Gen3.
- Not cheap CPU coolers.
- Very cheap used Xeons W available. Something like W-2123 (4C 8T).
- No IPMI.
- Perhaps not that great with low power consumption.
While you didn’t say that you are on a budget, I read it like this. Because locations with limited hardware choices are actually not that limited, but the prices are just nuts. I can be wrong…