Ah, ok, I misunderstood that this will inherit by session since I thought kwin start the script exit-with-session.

But then I still have one minor concern, for any window that actually inside kwin (e.g. right click on title bar -> window specific settings), the input method will be forced to qtvirtualkeyboard. Though I know input method is not commonly being used there, but I don't think that's expected behavior for input method user.