In general, when using a TV in place of a monitor, there be dragons. 
TVs like to do all sorts of weird things that make them not work as well as a real monitor, and you just ran into one of them: overscan.
You should set the TV to its native 1920x1080 resolution and then change the overscan value somewhere value to make the TV not cut off content like this. Some TVs have this option built into their settings or menu structure somewhere. There is also an option for this in the Display and Monitor page in System Settings. I think it’s only visible when in a Plasma Wayland session though. So if you don’t see it there, you’ll need to adjust it on the TV itself.
As for Virtual Desktops, Plasma doesn’t have per-screen virtual desktops like macOS and GNOME have.