@jdaviescoates Hello
I don't know if this is much help, but I have landed on using the following procedure:
First, on my Cloudron box, I mount my library as a shared volume. I then sync this volume to the Calibre web app using the following syntax:
rsync -azP --delete /mnt/volumes/<volumeid>/ /home/yellowtent/appsdata/<appid>/data/libraryNotice the trailing slash on the source, but not on destination.
The drawback on this method is that changes I make in the Calibre web app are overwritten when doing an rsync. So I try to not use the web app for changes (uploads, metadata edits etc).
More on this on this topic: Calibre not able to write on mounted (cifs) volume