There's a lineageos template for Android TV. I don't think grapheneos will ever run on something like that (it doesn't even run on phones with ten times the security capabilities of TV SoCs) but alternative ROMs are available. There's also KDE Plasma if you want to go the non-Android route, though you'll struggle to find good support for that.

One problem with that approach is that you'll lose access to DRM'd contents, so while the official Netflix/HBO/Prime apps will install on lineageos, their video quality will be terrible or they will refuse to work.

There are a bunch of Google TV variants (brands like TCL and Philips) that will let you turn on "basic TV mode" (https://support.google.com/googletv/answer/10408998?hl=en), disabling pretty much everything other than displaying content.

As for why the Chinese TVs don't have a dumb mode, I think it's because the Chinese market is full of devices crammed to the brim with smart features, so smart TVs are sort of expected these days.