Can you use Tailscale to join a Synology Directory Server domain remotely?
I have not used it, but it should work for that. Tailscale is just an overlay network VPN so it should not really matter what data you transport across it.
Would this be better than a site to site vpn?
Will it connect a workstation to tailscale prior to logging in so that the mapped drives policy takes effect?
I am not sure the start order, also mapped drives work poorly over a VPN.