Photos "curation" process doesn't load all (or in separate case, any) photos from the pre-existing Photos System photolibrary.

After an upgrade to Monterey from Mojave, Photos won't load my complete photolibrary. Upon first start of Photos under Monterey, it went through a lengthy "curating" process that had to be interrupted. Now the library is missing at least the last two years of albums, videos, and imported photos (nothing after Dec. 2020). I don't know if other earlier albums/photos are missing too. I had both a TimeMachine backup and a manual backup to an external drive before the OS upgrade. However, under the new OS, TimeMachine won't recognize backups made under Mojave, even though my user credentials remain the same, and simply replacing the Monterey mistake with the manual backup copy of my original photolibrary in the current Pictures folder doesn't solve the problem - still no photos/albums/videos after Dec. 2020. None of the Photos Help guidance to restore or repair a library helps in this case. Is there a way to clear Photo's "memory" so that the initial curation process can be repeated as if the first one, that was interrupted, never happened? Any other suggestions? Incidentally, I've just discovered a similar problem with Photos on a different computer that was upgraded from Catalina to Big Sur, except here the Photos curation process (which was not interrupted) doesn't load ANY photos, albums, or videos from that computer's System Photo Library.