Somehow I don't think it's a good idea to let Tailscale handle service authentication at all. It's supposed to be an additional security layer around existing infrastructure and services.
Letting Tailscale handle VPN and SSH at the same time is basically once again a single point of failure. Similar to exposing SSH directly to the internet.
I agree that this is how you should use it, but it is not "supposed" to be that in the sense that Tailscale strongly encourages you to use it in lieu of auth...