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)

The Picassa files are easy enough to locate in your Mac Finder, and then you can move them into a compatible photo editor. Another route is use the import function in Mac Photos, and bring the files across. Deleting the old Picassa app might be more difficult.it seems that you need to delete the 32 Bit apps before you upgrade.

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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.