Nice. Have fun with it!

Agreed. My answer was in the assumption that it’s about desktop apps as the OP mentioned Electron.

I think I heared that lan-only mode means you need to transfer the files to print via SD card.

Go with one of the ready to use systems. Flatpak, Snap, AppImage. Snap is largely Ubuntu Ecosystem, Flatpak is independent. AppImage is an option if you do not need/want a Sandbox.

Stay away from Docker and LXC for this use case (graphical applications), they are much more work to get going.

Hmm, good question. I think it is no problem. From an electrical point of view I think it would make sense that the relais power cycles when the plug reboots after an update. But I have not yet seen it do it.

I have the need for power metering only too.

I disabled the switch button on the plug so that they cannot be turned off by mistake. And set it to power on after power outage. Had no problems for a long time with this setup.

I have some Sandy Bridge systems here running strong as Linux desktops for light work. You know, these 4-core 3,3GHz processors from - hmm - 2014?