This would eventually allow KWin effects to react on that and e.g. only do an animation on startup but not when plasma crashed and restarted.

the propertieschanged signal doesn't seem to be emitted :/

I just figured, wouldn't a plain signal work? startupCompleted and kwin connects to it? Kwin just needs to know that plasma started once and then doesn't care if it later restarted etc? Dunno.