@rysiek there's a fine line between being cautious and being paranoid.
Right now there are loads of models of non-smart TVs. But they can receive firmware updates OTA. Do you run a firewall on your antenna?
There are hundreds of smart TVs which work even if they're not connected. You can de-solder their WiFi if you're worried.
If a thing you buy doesn't work - take it back. If it stops working, complain.
Finally, if you're that concerned, build your own.