Another Sony+Apple TV user here. Have had a couple of different Sony TVs, and if you keep them offline and never specifically request the smart TV UI (button on remote), they act almost indistinguishably from dumb TVs.

They also run near-stock Android TV so even if I decided to use the built in smarts, the experience probably wouldn't be too bad. They do come with a content ID daemon preinstalled, but that's easy enough to remove with a laptop connected via USB with ADB.

Additionally, Sony has been great about supporting firmware updates via USB flash drive, so you're not missing out on fixes and improvements by opting to keep it offline.