I have over 3,500 photos stored in iCloud. I have uploaded them all to a gallery on SmugMug using the smuggy app.
Finding a particular photo was getting harder so I created galleries for each year, from 2009 to 2017 on SmugMug. Uploaded each years photos to the relevant gallery on SmugMug then downloaded each gallery to my laptop, I left the 2017 gallery until the end of the year. It took a little time to get all of them uploaded and im sure there was a better way but its done now.
I don't use iPhoto and didn't look at copying the photos out of iPhoto but the above worked for me.
All new photos are uploaded to the years gallery and I can dowload the gallery at the end of the year to my mac.