A MacOS app to backup All Photos in iCloud to External Drive
You could do it in the Photos apps. If you launch it holding the option key, and choose to create a new library on the external drive. Then change the preferences to download all photos. It will take a while but that will download a full version of the library to the external. Then quit, safely remove the backup drive. Open again and choose the old local library, and quickly change the preference back to "optimize". It's a bit of a process but it should work.