Downloading iCloud Photos and Importing into Photos

You can get an exact copy of your iCloud Photos Library by creating a new empty Library, making it the System Library, and connecting to iCloud. Then all iCloud Photos stuff will be copied to it. Then you'd have two Libraries-- you might need a new drive. With such a large Library, this could take a week or two. Keep the computer plugged in and on with the Photos app in the background, and a good internet connection.

You can then delete every thing off Library 2 that's already on Library 1. You could even do some deletions while the pictures are downloading. Those pictures will also be deleted from iCloud Photos Library, giving you more room.

You can use Library 1 locally, but you can't switch back and forth with iCloud.

Many of us keep a Favorites Library that is the one we connect to iCloud. These are pictures I want to share with friends and family, passing around the iPad. The older or less sharable pictures are on separate archival Libraries. We use the trusted 3rd Party app, PowerPhotos ($30) to manage multiple Libraries.

You should "Download Originals" rather than use "Optimize Storage" so that you can backup these Libraries.

Can you off load some of them to other Cloud solutions until such time as you can back them up to a external SSD, since you seem to have access to a computer.