I don't know if this is your specific issue, but in my experience tvOS has always had storage management issues. Totally deleting apps won't actually move them from the available storage right away, and neither will disabling Aerial Screensavers. tvOS seems to cache a lot of data as well, and it doesn't automatically remove the cached data when the user actually needs more space.
But, I have noticed that if I let my ATV sit for a while with the offloaded/deleted apps, and/or disabled Aerials, the OS will finally behave correctly and allow downloading of new apps, or maybe the beta OS like you are trying to do.
Sometimes it needs to sit for a few hours, maybe even a day or two.
Worth giving it a try.