I think that’s how it works. I’m not 100% sure but I remember it being explained that once a user visits a community from another instance for the first time a copy is made and only new posts start to populate the copy. Meaning the past discussion is not ported. Also, I assume the users are low because it shows people subscribed in that copy just made but not including the other instances’ subscribers.
FireTV has a bunch of ads and it seems harder to remove on the newer models / firmwares. For Android, I would suggest something with Android TV like Chromecast with Google TV or Onn Android TV. I have no experience with apple tv so I can’t be sure.