A lot of Google/Android TV devices support "Basic TV" mode. You get the option during device setup, you can switch to it later by resetting the device. I would probably also not connect it to the internet, but that should cover making it a dumb device. I bought a Hisense one, tried it with Android TV for a bit, experienced it slow down and freeze up a bunch, and just switched it to Basic TV and plugged in a chromecast. Has worked fine since then.