I think that an “all the way” profile implementation can provide this functionality. There are other threads about profiles, like the “Party Mode” thread, but there is no mention of profile specific folders.
When configuring a music folder there can be an option to select the profiles that have access to this folder (only an admin can configure this). With profile information in the Roon database, non-accessible folders can automatically be filtered out when displaying e.g. albums. There shouldn't be a need for a separate database.
Something similar can be used for Tidal/Qobuz import. An import is only for the active user unless the user is privileged to add for others.
A system wide profile feature can be hard to implement in an existing code base but, in my opinion, it would be a great addition.