Hi folks, I've posted here before looking deeper into this problem, and as one suggested, I did reach out to Apple (without response from them 🙃). Anyway, I'VE DISCOVERED THE ULTIMATE FIX TO THIS ISSUE. Select all photos and hit the rotate button. (It will take the program maybe 20 minutes to process this request depending on how many thousands of photos you have. The program will also crash once it's done; but when you reopen Photos, all the photos will be rotated.) Finally, "select all" again, and [option]/rotate back to their original orientation. (The program will take another 20 minutes and another crash to process this; but once you reopen,) the Photos app will now be able to share/export all files like normal. (Hooray!)
[Edited by Moderator]