using a meshVPN like tailscale or netbird would another option as well. it would allow you to use proper backup software like restic or whatever, and with tailscale on both devices, it would allow restic to be able to find the pi device even if the other person moved to a new house. (although a pi with ethernet would be preferable so all they have to do is plug it in to their new network and everything would be good. if it was a pi zero then someone would have to update the wifi password)