This is not going to work on X11, where we don't use KWinDisplayBrightness (due to lack of Wayland integration) a.k.a. kwin: displayId prefix. Let's try to do a better job at fallbacks and matching displays based on matching KScreen::Output objects.
Through KScreen::Output, we can match EDID and isInternal properties against the display object, and then the output->name() will correspond to activeOutputName.