Photos.app progress when going from 'optimise Mac storage' to 'download originals to this Mac'?

There is no documented way to know when all originals have been downloaded to your Mac.

The Photos.app has no indicator for the state of downloading the originals. when I want to check, if all originals have been downloaded, I make a copy of the Photos Library on an external volume and open it in Photos.

If there are still missing originals, the photos may show white thumbnails, and Photos will give a warning that incomplete items need to be removed. Then I know, that I have to wait a bit longer.

Only one library at a time can sync with iCloud. If you duplicate the iCloud Photos Library the copy cannot access iCloud Photos, unless you make the copy your iCloud Photos Library. So the copy will be incomplete, if not all originals have been downloaded. When I make a copy of my current library to have a backup, I am checking immediately, if the copy is complete. Otherwise the backup would be useless.