Keep your tv disconnected from the internet completely like you did, and get a dedicated android box like the nvidia sheild for example. You can pick these up second hand for cheap.
Then uninstall any apps you do not require from the Sheild and you should be okay.
Now if you hold your own 4K media collection on a NAS you can install something like Plex and/or Jellyfin and keep your Nvidia Sheild completely offline only to access your local content. Though this really depends on your desired threat level or privacy level.
Agreed the Nvidia shield in its later updates to the OS really went heavy on the ads.
One way around this is with a OS downgrade and sticking with OS version 8.2.3 and not upgrading the home launcher, or replacing it with another launcher internally.
These were the steps I followed to downgrade all my Nvidia shield around the house.
https://florisse.nl/shield-downgrade/
Definitely on the lookout for a new media box though and the one you mentioned caught my eye as well.