I think I saw someone recently post that they use shortcut keys to switch to different libraries - but I can't find anything to do that - so perhaps I dreamt it - or they know calibre better than me, which wouldn't be hard
And have you ever had a look at this
Managing subgroups of books, for example “genre” ?
I have two permanent libraries. One with ~20,000 work related publications; mainly academic & govt papers etc, journal articles, media clips, press releases etc, just a few ebooks of historical interest. The other with a about a hundred non-work related e- books.
The work related library was originally multiple libraries somewhat like yours, but I found there were things I wanted to do that I couldn't. That was because the core features of Calibre are restricted to working on one library at any one time. So I now have them all in one library using a hierarchical subgroups as described in the link above.
I find its much less work having one library - eg I only define a tag once.
BR