I did some “preliminary research” some months ago as I wanted to update my current homelab with something else that was low power and also supported ECC. What I found was not good enough for my needs so I didn’t pull the trigger, although it can be useful for others.
The only thing I found that allowed me to go significantly lower than AM4 in size and power was going for an Intel mini PC/SBC that supports IBECC (in-band ECC)
and here on our forum
which is a newish feature that allows memory controller to run non-ECC ram as ECC (by eating up some of the RAM space for checksums, which is why “real ECC” ram sticks have more RAM chips than their capacity would require).
It seems some industrial pcs from Asrock have it and also the Odroid H4 has it
https://forum.odroid.com/viewtopic.php?f=171&t=48377
the Odroid H4 Plus does have pretty low idle power numbers, like low single digit with an Intel N97 or a Intel N305 processor
while it has only 4 sata ports, one of the bios images they offer allows you to install a board that splits the single m.2 slot into two x2 M.2 slots or into four x1 m.2 slots, or in a board with 4 intel 2.5G ethernet controllers, which I do find very neat
This is done by actually configuring the CPU’s PCIe lanes to be split in a different way, this isn’t bifurcation (a feature you can set up) nor it is using pcie switches/repeaters.
and similar to what other chinese mini PCs like the CWWK x86-P5 (here in bare form, there is also a version inside a case), that use the same hardware but don’t seem to provide the IBECC feature.