I’ve got ethernet running through the house, without running it through the walls. I’ve used cord covers but it’s still a bit tacky looking, TBH.
It’s totally feisable to do runs through the walls via the attic (my house is a single story on a concrete slab) but I don’t want to do that particular job. Might pay someone to do it some day, but it’s pretty low on list of things to throw money at.
Answering on the idea of port forwarding. Though like others have said, tailscale or similar is a better option.
Opening an ssh port would allow file transfer. You could also so remote desktop with port forwarding over ssh and vnc (or some other more modern protocol, haven’t futzed with anything like that in a long time).
Probably doesn’t need saying but make sure your system is up to date. Also using a nonstandard port and using key based authentication and disallowing password authentication and good security measures. If the device you’re connecting from isn’t something you’re bringing with you, you’d also need a copy of your private key.