[Adoby]

One nice way to set it up could be to have three copies of your calibre library.

Copy A is the main copy that you add books to and fiddle with metadata. You use it from the GUI as normal.

Copy B is a backup copy of A. You can use a sync utility to update it from A, as needed. Or automatically every time you quit calibre, by editing the script that is used to launch calibre. Preferably B is on a network share on a NAS or some other computer that is on 24/7.

Using a sync utility makes it extremely fast to sync any changes.

Copy C is a copy of B. It is also used by the calibre content server. It can be updated regularly by a sync task that is run at predetermined times by the OS, or manually by restarting the calibre server. The sync from B to C could be run every time the calibre server is started, in the script.

By having the extra B copy you can start and stop the main GUI and the calibre server independently, without having to be careful about not syncing and changing the calibre library when another instance of calibre is running. Also you get a nice backup system. Could extend it to keep more versions.