Thank you all for the incredible enthusiasm and helpful replies so far. I have learnt a lot thanks to you.
humdinger - I can confirm ZooKeeper works fine for running my script as an add-on, so there is actually no need for ‘ShellIt’ anymore.
extrowerk - I agree it could be done more efficiently, I just took the quick / lazy (and perhaps safe) approach for now. I found the wallpaper at https://www.goodfon.com/wallpaper/babochka-list-cvetok-goluboy.html
vidrep - I didn’t know Album handled attributes. I like the way you can drag-n-drop any EXIF tag, but have not found a convenient way to handle lots of images quickly. But seems there is a lot of potental there.
PulkoMandy - The top priority would be to implement what you suggest, so each copy could run its own script. This is actually how ZooKeeper works. I have not found a way to get the name / info for the current add-on, but it should be doable. I would also like to add a progressbar as I would run this on many hundred images at a time. I may play around a bit just to learn, even though we have ZooKeeper.
And the discussion around image thumbnails is really interesting, it seems jscipione is using attributes to store the actual image data, which is perfect. It would be fantastic if we could have a thumbnail view that could also display additional attributes below the image, like rating.