KWin provides shortcuts to send windows to specific screens ("Window to Screen N") and to set focus on specific screens when "Separate screen focus" is enabled ("Switch to Screen N"). However, the ordering of the screens is not predictable or consistent across different screen configurations.

This KWin script adds shortcuts with the same name and functionality, but with predictable screen ordering from left to right. So no matter what your screen configuration is, triggering "Window to Screen 0" will always send to window to the leftmost screen.

Ultimately, I think this should be fixed in KWin, but for now a script does the trick.