> Is there any smart TV that I can actually just use a TV how I want?

I’ve heard that large computer monitors and TVs intended to be used as displays can be used without connecting them to a network.

> Or am I reduced to buying an Apple TV device and unplugging the TV from the internet entirely ?

An Apple TV is a good choice even otherwise. I’ve never seen a smoother and quicker interface on a native Smart TV (granted that I’ve only seen Android and webOS). I use my Apple TV as the only network connected device while my TV is not connected to any network ever. Once in a while, I update the TV’s firmware by downloading it to a thumb drive and plugging that into the USB port of the TV.