After disabling iCloud Photo Library, cannot re-enable due to insufficient iCloud Storage
A suggestions I've seen on other Q&amp;A forums is to use another device with iCloud Photo Library enabled and wipe the Library so there is enough space for all your photos to be re-added.

If you don't have another device with iCloud Photo Library, I believe you can delete the files from General > iCloud > Storage > Manage on an iDevice.