Set an environment variable named CALIBRE_OVERRIDE_DATABASE_PATH to a local path for your metadata.db file, and leave the library location in Preferences set to your NAS share. Calibre looks for that environment variable (and many others) to override default behavior.
I've had my database and library separated for some time now, for similar reasons to yours (I use a WHS rather than a NAS, though I do keep my metadata.db file on the WHS just in a separate location).
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