In my continuing quest to ensure that we have local copies of all cloud data like photos, I re-discovered that we had about 11,000 pictures in Apple Photos from when we had an iPhone in the family.
iCloud Photos Downloader (icloudpd) is a command line utility which uses Apple's API to download your files from Apple Photos. Apple's API seems quite good, one can download the original resolution files without restriction or limit. icloudpd prints each filename to stdout as it runs. It took about 5 hours to download 11,000 photos and videos, 376 GBytes of data altogether.
Once downloaded to the local filesystem, immich-go imported the files to immich while suppressing duplicates as it went. Of the downloaded files, less than half were new. The rest had apparently made their way to Google Photos at some point, and already imported into immich.
As an added bonus with the photos deleted, we no longer need to pay Apple for so much storage and were able to drop down to a less expensive tier. We now have just enough to back up the iPads.