As convenient as it might be, the most secure smart TV might be one that isn’t connected to the internet at all.
Yep. Just don’t. Most smart TVs are going to come with a USB port that you can use to run firmware updates. Want to stream? Get an inexpensive Roku or FireStick (which still might spy on you), or fork over the big bucks for an Apple TV.
And if you have previously connected a smart TV to the internet, not only would I urge severing the wireless connection, but also changing your home wifi password as there’s no guarantee that a) the password hasn’t been transferred upstream (unlikely though that might be) and b) that the TV doesn’t continue to hold onto the password and connect to your wireless anyway. USA Today ran a story about smart TVs and “post-purchase monetization” back in February 2019 that’s well worth the read. As soon as monetization is involved, security concerns go right out the window.