@zamundaaa: I recognize that the notion of an "external brightness change" is less true for (this current implementation of) KWinDisplayBrightness than for backlight or DDC/CI implementations.
As long as KWin only emits a single output change signal to the target (even if it animates the brightness), then we're okay because externalBrightnessChangeObserved signals get ignored if the value is the same as the one from the last ScreenBrightnessController::setBrightness() call.
Animated or not, delayed notifications of potentially old values present the same problem that I'm still trying to fix property for judder-y backlight brightness slider controls. We can't just stop observing brightness changes or we might miss an actual external one, but we also have to expect that a new setBrightness() will come in earlier than the delayed change notification. It would be neat if KScreen allowed me to associate a context string to filter out notifications for the change that I myself have requested. Delayed error handling is fine, delayed confirmation combined with high-frequency user input is hard.