The problem is, the iPhone user guide for iOS 12 says 'My Photo Stream is automatically turned off if you use iCloud Photos.' So it will probably not upload to My Photo Stream, even if you turn it on again, as long as you are using iCloud Photos on the iPhone. (https://help.apple.com/iphone/12/#/iphbfeb468fchttps://help.apple.com/iphone/12/#/iphbfeb468fc)
Since all photos did upload to iCloud, you could create a new, empty Photos Library and enable it for iCloud. The Photos from iCloud will download to your Mac, without uploading all photos from your Mac library to iCloud. Then you can export all photos from the new library with 'File Export Export unmodified Original' and save them to your. In two ways: (1) by making sure that iPhoto before 9.6.1 was not going to launch under the newest Mac OSes; and (2) by removing version 9.6.1, the last update, from the App Store so that it could not be downloaded.
Google Photos App Mac
Mac Apps Download
Upgrade Photos App Mac Download
Apple Photos App Download
You could simply try use iCloud Photos on your Mac as well. Since all photos did upload to iCloud, you could create a new, empty Photos Library and enable it for iCloud. The Photos from iCloud will download to your Mac, without uploading all photos from your Mac library to iCloud. Then you can export all photos from the new library with 'File > Export > Export unmodified Original' and save them to your main Photos Library.