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.
Not sure about the odroid but I got myself the nas kit from friendly elec. With the largest ram it was about 150 bucks and comes with 2,5g ethernet and 4 NVME slots. No fan and keeps fairly cool even under load.
Running it with encrypted zfs volumes and even with a 5bay 3.5 Inch HDD dock attached via USB
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.
You can get a 1 -> 4 M.2 adapter for these as well which would give each one a 1x PCIe lane (same as all these other boards). If you still want spinning rust, these also have built-in power for those and SATA ports so you only need a 12-19v power supply. No idea why these aren't more popular as a basis for a NAS.
No IPMI and not very many NVME slots. So I think you're right that a good mATX board could be better.