Software to synchronize mobile SSD to network disk

I recently moved many photos to an external disk attached to a Time Capsule. This makes the photos available to all users on the local network. The issue is that one user has the photos on an SSD and might edit a photo at a remote location. Also, I might edit (but more likely add or delete) a photo on the disk attached to the Time Capsule. What is the best way to ensure that changes get promulgated in both directions?

No one here uses iCloud for photo storage, nor Photos for picture editing or managing. Managing backups of the photos is a separate issue that I believe I have under control. It is making sure that updated pictures move in both directions that is the challenge.

Thanks for any insight.