Update from my end, I went ahead and installed TrueNAS. I originally planned on going with OpenMediaVault and just mounting my mdadm array in there and being done with it. Turns out that proxmox requires a disk wipe (or a probably fragile custom fstab mount) to be able to pass the disks through.
So I decided to try running OMV bare metal, but the i226 NICs aren't recognised at install.
Since I'd have to wipe the disks for proxmox anyway, or do the alternate OMV isntall atop Debian I said sod it and went with TrueNAS which is working for now. Mostly.
Power use is still a bit of an issue but I've mostly explored all my options. I ended up scrapping my MWE400 just like
@bugmenot and swapping it for the picopsu that was powering my old homelab. This saved around 2W of power. I'm not too worried about surge current since I'm running only 3 drives and the picopsu
claims to be 250W. I'm powering it with a decent quality 90W brick.
The powertop is something that's being weird for me. Running powertop more than once will cause the system to go wildy unstable and eventually hang. I've no idea what's causing that but it's very frustrating.
powertop --auto-tune drops the idle power from around 32W to around 26W (with disks), so it's basically a requirement. But if I accidentally call it a second time the system freaks out. The TrueNAS web interface behaves weirdly, pages can still be browsed but nothing loads. And then eventually it goes to the " Waiting for Active TrueNAS controller to come up... " screen. Then eventually I lose connection to the SMB share I have and it never comes back (within the ~20 mins I waited).
I saw
similar behaviour with proxmox, where it would tell me I had put the password in incorectly even if I hadn't (copy paste). I think that powertop is clobbering some important services somehow if it's called again.
What's
really annoying is even the service has this behaviour. I set up the standard oneshot service to run on boot, but all that does is make it boot with that instability as though it's running the command repeatedly. Very weird. I messaged cwwk about this but since they never bothered to help me with the BIOS install I don't hold out much hope.
Also yeah, my HDMI port still isn't working. And if I don't boot with DP connected, I can't
then connect it later (black screen). I tried installing windows like
@chripo suggested but sadly it didn't fix it. It's not a major issue since I don't actually need the display. It would just be nice for debugging.
Right now I can't reccommend these boards. Once the instabilities are ironed out and there's proper documentation (other than this thread!), I think they will be a great buy. But not until then.