It looks like I can change from mastodon.is-a.horse to just is-a.horse by having the main domain just point at mastodon directly, instead of doing the "fun" nginx redirecting stuff for certs/webfinger, now that this is definitively the main use for the domain.
I'm guessing things will explode if I were to remove the mastodon subdomain, but aside from that, does anyone know if there's some other hidden problems I'm not seeing?