I haven’t used Plex in a decade and I use Jellyfin, what you’re describing sounds perfect. I read up a bit on STUN servers and it’s what Syncthing uses, but they also mantain discovery and relay servers (and anyone can host one and can be added to the public list). Security wise they seem to be doing fine?(I’m not an expert, just an informed user)

Idk what combo Jellyfin would benefit the most from; are relay servers needed? The workload is similar but probably higher on average, people stream more often than they do backups