Photos

Export photos from system photo library to a desired location say download folder in finder , then connect an external hard drive to the Mac and transfer the photos from download folders to the external hard drive , hence all your photo data is saved .

You can select few photos see this article Select and deselect photos in Photos on Mac - Apple Support and they can be deleted in system photo library folder , you should select them and delete to recently deleted folder see this article Delete photos or recover deleted ones in Photos on Mac - Apple Support ( its your choice to delete them permanently immediately or after 30 days the photos will be removed from the iCloud server ) .

In this way the complete photo data is still on external hard drive , but the space is reduced in photo library .

See this article to export photos Export photos, videos, and slideshows from Photos on Mac - Apple Support

Have you got Optimize Mac Storage enabled on your MB? I believe that will keep your free space to something like 20%.

No. The iCloud Photo Library is a syncing feature which since all changed, additions and deletions to all Macs and mobile devices signed into the same Apple ID and the iCloud Library.