Every time somebody brings this up, there's the same obvious answer. The only real option is to have an external streaming device and to disable or completely ignore the built-in operating system.

I got a Philips TV and just never connected it to the internet. Only ad I see is the ugly Netflix button on my remote, rest is handled by OSMC.

I think most TVs can do this. I've even discovered that Samsung that's notorious for forcing you to go through a network setup wizard can be bypassed just by pressing the right arrow button instead of selecting a WiFi. At least all the Samsung TVs from my old workplace had this bypass.

Finally got sick of the FireTV so I plugged a mini PC into a dumb TV. It's kinda sad to have to use a keyboard instead of a controller, but it was a minor tradeoff for a much better experience.