This is a very complicated topic and I don't completely agree with your proposal. The queue volume is the master volume for everything that is playing. Playlist volume is only relevant for a playlist and will be used to set the queue volume to when you load a playlist into the queue (after a currently playing song has finished).
What you propose is attaching the playlist volume to each song. But that would lead to the queue volume jumping up and down from song to song which is a horrible result in my opinion.
I agree with you that it is not intuitive that changing the volume of the currently playing playlist has no effect on playback. But I have not found a way to make this work so far for both operating modes (advanced queue mode on/off).