Apologies for not replying earlier - I seem to have missed some of the discussion. I agree with the structure proposed.
Suggest that “Collection” here is used to denote works which are related either by the composer (Wagner’s Ring Cycle), or by being frequently recorded/performed together (Bach’s Well Tempered Klavier Book 1).
IMO the use of this “Collection” relationship versus the Work/Series relationship needs clarification. Perhaps the latter should be reserved for Opus (Bach BWV series) and Published (Bach Anna Magdalene Notebook) collections, and renamed “Catalogued in” and “Published in”. I know that there will still be some anomalies, but it might help clear the fog.