What is the "typical" (WindowServer) O/S (CPU/MEMORY/THREAD) usage per "display" (1920 x 1080) by the process?

There is no such typical cpu/memory/threads "usage" per connected display.

As a quick test you can watch the values in Activity Monitor and then plug in a few displays to your computer, and then unplug them one by one again. You'll see that there's no fixed value that the usage jumps by. Nor is there going to be on a longer term.

All in all the memory usage by WindowServer depends on what each and every GUI application on your computer is actively doing - it is going to be very different over time.

The WindowServer responds to the demands of the user and OS to display all the graphical content on the screen. Its CPU and memory allocation will change depending on how many windows and dialogs you have open. Scrolling, or otherwise animating the display, will increase the load briefly.

The values you show are 'not untypical', certainly.