Photos stuck in a loop, “curating photos, analyzing scenes, curating favorites….”

That's amazing and scary! You seem to have thought of most things we would ask! If you haven't already, try these things:

• Restart the computer (of course)
• Start in Safe Mode (different for different computers) This bypasses certain potentially disruptive processes.

           Start up your Mac in safe mode - Apple Support 

• Make a new user and see if the same problem recurs with it. This tells you if the account has a problem
• Try another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens.
• Try again to rebuild your Photos Library by closing Photos and re-launching by option-command-clicking the app icon.

This will eliminate some of the sorts of things that can cause problems. It may fix it!

Let us know what happens!

Richard has already given you valuable advice. Just for completeness sake - the status message “curating photos, analyzing scenes, curating favorites….” is perfectly normal. It has been introduced with macOS 10.15 Catalina and you may be seeing it over and over again after a major system upgrade, when Photos needs to analyse your Photos Library again. If your library is large, Photos may need several weeks to complete this "curating". What is happenings that Photos is trying to find out what your photos are showing and picking the best shots for the layout of the tiled view in the "days". This has nothing to do with the problem, that your older photos seem to be missing.

Which system version did you upgrade from? Did you upgrade from a very old system version? I am astonished that you have not come across this message before.

What are you seeing below the "All Photos" tab? Does the status message there is showing the number of photos and videos plus some "Items" or only photos and videos? Any mention of "items" would be bad. It would mean that you are having incompatible media in your library, that would need to be removed.