Photos & iCloud

You can at least find the ones with uncommon file types by creating a Smart Album that excludes the file types you commonly use. Here's an example Smart Album that excludes a number of types I do use:

You could start with the top two or three at first, then add other types that you intentionally use. I continued until I covered all the types I use, as evidenced by the "0 items matched" shown. (All of the types shown there are recognized by Photos as photo or video formats.)

That won't show you which items might be damaged or otherwise unusable, but it might be a start.

What do you see at the bottom of the "All Photos" display? There should be a count of the photos, videos, and maybe other items. Below that, there should be a status indication, such as "Updated Just Now" (the usual steady state), "Updating..." or a progress count of uploading or downloading.

If you do not see that, check that the library you have open is designated as the System Photo Library. There is a "Use as System Photo Library" button in Photos > Preferences > General. If the button looks faded out, that's an indication that it is already the System Photo Library. If the "Use as System Photo Library" button has the same appearance as the "Show in Finder" button next to it, click. the "Use as System Photo Library" button, which should then fade to gray.

The "items" could be PDF files, photos or videos damaged beyond recognition or in very old formats, or leftovers from previous versions of the library.

Let us know how things are going in a few days.