I did basically what you are trying to do:

I installed Calibre docker on my server machine with the DB local (important because the DB won’t work over a share) and the book storage on the NAS.

Then I installed Calibre-web docker and pointed it to the same local DB and the same book storage on the NAS.

Now I can use Calibre for import, DRM removal, metadata updates, etc. And I use Calibre-web for user management, OPDS feed, etc.

Let me know if you want more info.