I had to give up on this goal. There are simply no dumb TVs in my country.

I even considered getting a monitor, but you can't get one as big as a TV.

I can recommend Sony TVs for this usage. They run very vanilla Android TV, won’t gripe at you about not being connected to the internet, and accept firmware updates via thumb drive. The newer ones also have a “simple TV” mode that pares back the TV’s “smarts”.

They’re not cheap, but that’s to be expected if they’re not heavily subsidized by ads and content analytics like many smart TVs are these days.

I can confirm the "won't gripe" part. Mine has been disconnected from Wifi for the last four years. It's been a great TV.