yes, usually people use immich here but photoprism should work too
If your friend is on the same network as you it should plenty easy for them to back up stuff as well
if they are on different networks, you would need to setup something like a reverse proxy for them.
this can work, just the ease of it is pretty variable. I believe this can be done easily with plexamp but some features are locked behind a paywall.
It does get a bit harder when you try to do this outside of plex
absolutely no idea, not something i have done myself or even looked into.
hexos is going to get built in vm support in the future.
at this time outside of plex and immich setup most things are done manually via the truenas interface
in the future more apps are planned to be able to install in 1 click