I also have a $900 monitor (provided from work) which is also a built in kvm switch, and it can show two desktops, one HDMI/windows and one usb-c/mac, side by side or as an inset as well. There's no delay switching either.

It is supposed to hot-switch the inputs if I move the mouse to the edge, but it does not, I guess it's because one of them is HDMI.

I used to have a Lenovo dock that I used as a switch, but not anymore and there's definitely less clutter.