@girish Thanks for the detailed response on the issues and limitations surrounding shared volumes. I'm basically just looking for anything that is an upgrade on SFTP and could be an easy way to store and access media across apps on Cloudron that support using shared volumes. And if possible to achieve this without disappearing down a trouble shooting rabbithole myself.
I would be happy to try using a volume that is shared between Jellyfin and Syncthing, if that's something that would work? Syncthing seems to have a good reputation for transferring larger files reliably, and has peformed pretty well on a few test I've tried.
I did read about the unreliability of Nextcloud external storage and issue with Lychee and creating it's own database (which I guess makes it a less desirable all purpose photo manager alternative to Google Photos).