Q: Archiving iCloud photo library to other media

What about using REWIND Arching Software -- http://blog.digistor.com/apples-new-photos-app-how-to-back-up-your-photos-withou t-icloud-part-1/

Couldn't you export the photos from iCloud to the iMac then proceed with the option above?

See this -- Archive or make copies of your iCloud data - Apple Support

If this is an iCloud Photo Library and you have a Mac with enough space, you can go to Photos > Preferences > iCloud and choose download originals. iCloud will put them all into your system library.

At iCloud.com, hold down the option/alt key while selecting the photos.When you get them all selected, you should see a download option.