You may also want to check for referenced items. That may be causing the same problem. Are there any items in a smart album with the rule "Photo is Referenced"?
Are you seeing any white thumbnails, when you are scrolling the All Photos tab?
If the smart albums cannot point out any problematic items, you will have to test systematically for items that cannot be processed by photos.
I would start with the most recently imported items, the ones you imported directly before iCloud could no longer upload all photos.
- Export small batches of videos and photos (the unmodified originals), then try to import them again to a different, empty Photos Library just for testing. Any item that Photos cannot import into a different library needs to be converted a more compatible format.
The usual suspects for incompatible items in your library are PDF files, older videos, images shared with you, scanned photos, photos downloaded from a website as a web link, or older videos, imported before the upgrade to macOS 10.15 Catalina, when Apple deprecated the support for some legacy image and video formats.