I think you're right generally, but I wanna call out the ODROID H4 models as an exception to a lot of what you said. They are mostly upgradable (SODIMM RAM, SATA ports, M.2 2280 slots), and it does support in-band ECC which kinda checks the ECC box. They've got a Mini-ITX adapter for $15 so it can fit into existing cases too.

No IPMI and not very many NVME slots. So I think you're right that a good mATX board could be better.