There are lots and lots of alternative calibre servers a bit of googling will find you dozens. If you want to run the official server, which is the most capable of the lot, then your best bet is to copy the db file of your NAS and run the server using that copy, with the CALIBRE_OVERRIDE_DATABASE_PATH env var (see
https://manual.calibre-ebook.com/cus...ment-variables)
But note that this will limit you to only exposing a single library witht he server and you must use the server in read only mode.
You can write a trivial script to stop the server, update metadata.db from the NAS and restart the server from cron.