So thinking ...
- Calibre Web can only use one library at a time
- custom metadata fields like @ownedbycats uses are easy
- so add a "book type" or similar category field instead of a second library, only sync proper "books" to the e-reader
- have everything available in a folder structure in Nextcloud (automatically, Calibre creates a folder structure - by Genre/Author sort/Title in my installation)
- also have everything available on a self hosted web app for easy tablet access
- ...
Looks like a plan.
P.S.
There's a ready to use FreeBSD port for it:
https://www.freshports.org/www/py-calibreweb/
I consider that a sign