FYI, unRAID is moving to a subscription, so if you want to try it out, it might be good now before they change.
Also, I thought I read that docker/portainer was possible on TrueNas. You might check it out if Kubernetes is giving you trouble. I might be wrong, but I can’t imagine why it wouldn’t work on the base Debian install.
I use CX file server. It can create an SFTP connection. I also have a VPN through my router.
Syncthing may be easier if you don’t have a VPN.