I wonder if Hazel can help here? I don’t use Photos on my Mac so have no experience with automation in that arena.
Here is a simple example of an iOS Shortcut you could use for this. Albeit manual, it does support the general use case described.
Note, I had to use iCloud as the example file store due to a bug in Shortcuts that is preventing me from getting to Dropbox. Simply modify that step to point to your correct Dropbox folder.
https://www.icloud.com/shortcuts/40b38c767ca14d1d924364e4ca748ec1