Exporting and re-importing pictures loses lots of information, including the organization, faceIDs, and especially the reversibility of edits. If that's what you really want to do, then you can choose File>Export>Export nn Photos and then
choose "Moment" as the subfolder. Photos will try to put pictures together that have similar "moments." You can try it and see if that works for you-- it's pretty good. But then you would need to export the Unmodified Originals which can't be organized that way. So you need two copies of every edited picture.
Personally, I make heavy use of Photos' ability to save space and and increase versatility by keeping pictures in multiple albums-- the picture of "Aunt Ethel at the Grand Canyon" can be in the "Aunt Ethel" album with other pictures of Aunt Ethel, and it can also be in the "Grand Canyon" album with pictures of other people at the Grand Canyon. And it can also be in a the "September 2015" album with other things that happened then. All that is lost with the Export and Re-import process.
But it's not clear why you would want to do that. You can just duplicate a Photos Library, and this will take up about half the space of what I outlined above. And it will have all the organization. If you're going to use an external drive, you can just drag the Library from the Pictures folder to the drive, and you'll have two identical Libraries. I did that, and then I erased all the non-favorite pictures from the System Library so my iCloud Library would be smaller and have only the pictures I really wanted to see and share on my iPad and iPhone.
I have a bunch of Libraries like Wife's Family, My Family, Work Pictures, Nikon, Phone Pics, etc. Favorites has the best of these. As Yer_Man suggests, I move them around using the trusted 3rd party app PowerPhotos ($40.)
What do you think?
