Tailscale now supports buying Mullvad subscription through them and using it with their app. So it solves this issue, although the control you have over your Mullvad tunnel is less and it misses some advanced Mullvad featurea. Still probably good wnough.

I ended up going a different route by buying a VPS and hosting Caddy as reverse proxy. Then I only have my VPS and NAS on tailscale and other devices use the reverse proxy.