I did the same with a Sony A80L, which also runs Google TV. I even uninstalled the bundled streaming apps for good measure, although I never see the home screen.
It behaves like a monitor. I never see the TV UI unless I ask for it.
My LG could be configured to behave like this (and believe me, I am thinking about going that way).
I have a cheap FireTV which cannot be made to behave this way. If you disconnect it from the internet, it will still require you to interact with the (slow-ass clunky) OS in order to select a different input.
Your best case scenario with some of these smart TV's is the ones which run Android to replace the launcher. Possibly, this gets reset periodically, meaning you have to keep doing it.
Apparently there's a few Fire devices which can be flashed with LineageOS - I might try researching that and see if it is doable. A FireTV stick with LineageOS would be the best case scenario.
I find that most of those reset to some nonsense occasionally or whenever the power goes out. I make sure they have no internet connectuon, but I usually have to dig up the remote to get back to hdmi1 so my device interface will come back up. I accept the annoyance because I accepted the discount that they give to have all of the spyware crap on there that I am blocking. I wish they could sell something cheaper that is just a display, buy product managers will be product managers.
It behaves like a monitor. I never see the TV UI unless I ask for it.