Allow Shell Script to Access Protected Library Folders (launchd)
I overcame the issue by creating an application wrapper for the script using Platypus - https://sveinbjorn.org/platypus.

I gave the resultant app Full Disk Access in the Privacy pref pane and this allowed it to run.

(Any subsequent change to the core script means regenerating the app and removing and re-adding it in Privacy as the new file is not recognised as having Full Disk Access.)