A DNS filter (Pihole) will only stop the TV from DNS resolution, and won't necessarily stop it from trying to phone home to some dodgy servers if the IP addresses of said dodgy servers are baked into the OS.
I don't fully understand why you are concerned about what the TV can access on WAN, and not about what the TV can access on LAN? Put it on its own subnet if you're worried about it sending information back about other devices on your network.
A DNS filter (Pihole) will only stop the TV from DNS resolution, and won't necessarily stop it from trying to phone home to some dodgy servers if the IP addresses of said dodgy servers are baked into the OS.
I don't fully understand why you are concerned about what the TV can access on WAN, and not about what the TV can access on LAN? Put it on its own subnet if you're worried about it sending information back about other devices on your network.