The solution works really well though - a reliable, name-brand SSD (sic) in an old $100 Dell Latitude as the workhorse. The Latitudes allow changing the charging profile in BIOS to "always plugged in" amongst the other power friendly things (disable TurboBoost, etc.). Built in UPS, keyboard, monitor, wifi/eth; portable, quiet and with the right tuning (disabling a lot of modules in GRUB or module blacklists) runs most of the time without kicking the fan on. I run 2, one doing all the work and another backing it up. I run with only a partition encrypted (requires manual unlock after boot, my choice) so that I can remote login after boot to mount that, never have to actually open the laptop unless, say, wifi goes south. Laptop dies (they never do), just pull the SSD and slap in another one it's Linux it'll boot.
I agree, especially about the integrated USP and peripherals.
In my case, I currently use a Macbook Air M1 (my personal one which I currently don't need as a laptop). Absolute overkill and in some aspects annoying - only two USB-C ports for example - but it was available and it's fanless and doesn't need much power.
What storage enclosure (Thunderbolt?) and network filesystem do you use?
When there's a MacOS security update and the Macbook reboots, do NAS functions work before a local user has logged in?
Just a random powered USB-C Hub with a few external drives on one port and a Thunderbolt SSD I had from an old project on the other. For now, I just use the SMB server built into MacOS because I've not gotten around to installing Asahi on it.
I think I turned auto update off on this machine but if it reboots, you would have to login first. Doesn't bother me, though. I don't have any uptime requirements.
You can fully control when macOS does updates of any kind with a couple of UI toggles.
Any launch daemon service will load and/or start at boot, only user-domain services require a login (like the Dock or menu bar would)