I have been in similar situations before and there is no easy answer
I have no idea how Peertube works but if you're on Linux you might be able to turn on filesystem compression.... but if the data you're storing is already compressed (like video) that probably won't help very much
You could try sshfs like you mentioned in your post but it might be unreliable :( , maybe you could try NFS or even mount remote storage over Plan 9 protocol
It seems you need some kind of NAT traversal mechanism. Wireguard is pretty easy to set up, it's built-into the Linux kernel these days. I've heard good things about Tailscale as well.
I just setup tailscale on my home Ubuntu jellyfin server the other day for the first time and it was a totally painless experience. Been working great.🤞