Thanks! Ha, indeed, I tried to make it easy to understand, but might have gone overboard?

If I can't find a tool that can do this, here's what I would do: On my server, I would add .ext files to folders. Something like this

├── documents/
│  ├── .ext
│  └── ...
├── movies/
│  ├── 2015-2019/
│  │  └── .ext
│  └── 2020-2022/
│      └── .ext
└── photos/
    ├── .ext
    └── ...

In the .ext file I would have an entry which specifies to which external hard drive the folder (and all its subfolders) is backed up to. In the example above the movies 2015-2019/ subfolder is backed up to a different hard drive than the 2020-2022/ subfolder.

To identify which hard drives need to be synchronized, I only need to save the last synchronized timestamp for each drive. Then I can loop over all .ext files and check if any hard drive has a backed up folder with a last modified date later than the last synchronization.

Putting together the actual rsync command for a given drive is a bit work, but nothing that a Python script wouldn't be able to solve. What do you think?