Mhh, you are probably right. I'm really hesitant to invest more time into troubleshooting tailscale. My own tailscale connections at home are stable, and for troubleshooting at my family member I always have to drive and setup my notebook there.
Maybe I could try to setup wireguard directly in the fritzbox router at my family member. That might need less maintenance as running tailscale in a docker container, as it would remove the wireguard connection out of the NAS itself.
Opening a port directly gives me kind of an ick, because I don't want to fiddle too much with the family members network. If something happens with it, I will be the one troubleshooting it, and I don't need more work currently.