If you are OK with external USB drives, I did just that with my Atomic PI [1] (Atom-based, 35$ SBC, 3.5W idle, running Debian on its internal EMMC, with two external USB drivers in ZFS-mirror configuration).

[1] https://www.thanassis.space/atomicpi.html

You can also get rackmount servers in 1U format with Atom CPU's that support 4 3.5" drives, e.g. https://www.broadberry.co.uk/intel-atom-rackmount-servers/cy...

Also plenty of slightly more capable laptop class SKUs that are in this ballpark.

Can you still find "desktop" motherboards to go with them?

A long time ago, I remember there being desktop MBs for the Pentium M.

For an always running thing, choosing a laptop motherboard is a nice way to guarantee all the power saving features will work properly.

Most desktop motherboards won't allow the PCI express bus to sleep for example, using an extra ~4 watts, $8 per year in eco-friendly countries. Across the ~10 years your NAS will probably sit in a cupboard, that's $80, which isn't an awful lot, but probably would have let you buy a bigger SSD or hard drive which would have had more utility.

That's true, but then again I wouldn't be comfortable with a single-disk NAS...

However, for running random VMs, etc, I think a laptop is quite a good proposition.