LFS + twm FTW.
You seem to have a good sense of what disros are out there. I think any beginner friendly distro with wherever user friendly DE it comes with will probably be fine.
I’d say maybe stick with a deb or rpm one which most are anyway, for the compatibility point you already made. If you/they need something that’s otherwise not available in your distro’s repos, there’s probably already packages or third party repos available.
But the ask is a recommendation. I find myself generally recommending Fedora, despite not having used it myself for years. It’s easy to install, you can choose your DE, it’s stable, has a good release cadence, and is easy to upgrade and maintain.
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.