photos not importing into imovie

Optimized storage is the problem. As you have found, if you turn it off the cloud will empty all of the photos that it is storing onto your drive. You have a very large amount of photos, and it would take a long down to complete the full download, maybe a couple of days. Besides, I doubt that you would want all those photos taking up space on your drive, or you wouldn't have used optimized storage in the first place.

I think that what you can do is open the Photos app library, create a smart album with a rule that encompasses the parameters that you want, and then open the photos to download them to the Photos library. You woudln't be downloading the whole library, but just those photos that you selected. Then you should be able to export the opened (downloaded) photos to your desktop, or import them into iMovie. Not using optimized storage myself, I think that that's the way it would work.

-- Rich

Are these photos stored in the cloud? If so, you may be mporting low res thumbnails instead of the full sized photos. If you think that this might be your situation, try first downloading the photos from the cloud, and then importing to iMovie.

If by chance you are using iCloud Photos with optimized storage activated, that would cause some of your photos to be stored in the cloud in order to save space.

-- Rich

Try dragging the pictures from your Photos app onto the desktop (or somewhere convenient) and open them with Preview to check they are OK.

Then import them into iMovie using the Import window.

The latest version of Final Cut Pro has problems importing photos and it is quite possible iMovie could be suffering from the same bug.