I have on occasion added the hits tag when the artist is reaching a different milestone like "downloads", "followers", etc. Should it be used like that, and if so, would it be preferable to alias it to a more general term? I notice there is a milestone tag but it confused me a bit at first until I realized it's indicating a game company at the moment (but it's not set as a copyright yet).

artist_milestone maybe? That way it's clear what the intent is from the get-go.