Unable to sign Photos back into iCloud after sign-out

MacMiniAndMore wrote:

.. I have 80GB storage left on the iCloud my Photos Library file is 97GB. Does iCloud assume this is an entirely new set of photos?

That is exactly what is happening. Photos is using the worst case assumption, that all photos in your current Photos Library are different from the ones already in iCloud.

The quickest way to sign back into iCloud Photos would be to save your current Photos Library on an external volume and then create a new, empty library in your Pictures folder ( Create additional libraries). Enable this empty library as your System Photos Library in "Photos > Preferences > General" and then as the iCloud Photos Library in "Photos > Preferences > iCloud" . This will avoid a new, lengthy upload. The photos will just download from iCloud to the new, empty library. The only draw-back would be that the projects will be missing. This is why you need to save the old library.

I forgot - if you do not start with an empty library you would indeed have to purchase more cloud storage, temporarily, so you can enable iCloud Photos again. Once the library is in sync, you can cancel the additional cloud storage, as the library will be as large as before. But it will take much longer than deleting the local library and just downloading the photos back from iCloud to an empty library, because iCloud will upload the complete library from your Mac and merge it into the existing library.