First thing I'd probably consider is something like OneNote or Evernote. they are good at managing loosely structured data, they can handle multi-user sharing, they run on Windows, MacOS, Android and IOS (not Linux afaik).
For concurrent multi-user access I think they are tied to their respective vendors cloud storage offerings.
If you want to use calibre you ought to look at the CalibreSpy plugin which can be used as a read only app, so it is safer for concurrent multi-user access to a common library.
BR
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