Suggesting to combine Playlists, Qobuz (and other services) favorites and local Library favorites all into Favorites. It seems that this is not available. At least I have not stumbled over it yet.
Although, personally, I’m not that much into playlists, ideally it even had combined Playlists of all.
In addition to the high-end technique of Audirvana behind the scene. The free combination of items of all content channels would be the usability heaven I think.
Certainly, am I aware about the challenges for a programmer of synching flags across content channels.