I want to reduce wasteful power consumption.

But I also desire ECC for stability and data corruption avoidance, and hardware redundancy for failures (Which have actually happened!!)

Begrudgingly I'm using dell rack mount servers. For the most part they work really well, stupid easy to service, unified remote management, lotssss of room for memory, thick PCIe lane counts, stupid cheap 2nd hand RAM, and stable.

But they waste ~100 watts of power per device though... That stuff ads up, even if we have incredibly cheap power.

I bought a used Coffee Lake era Xeon 2224G workstation with 32GB of ECC RAM to use as a NAS. It uses 15 Watts at the wall measured with a killawatt while streaming 4K with Plex.

Nah nah I disagree. My i7 8700k old PC server runs around 60 watts with 4 hard disks and ~30 running containers. It's a large machine so that I can easily expand with more drives but I can easily buy a smaller mobo on the used market if I wanted something smaller. Depending on how old your NAS is, and what you are doing with it, PC may be more power efficient.