Same exact issue happening to me.
In the past it has found more photos than just the new ones (and doesn't seem to remember which ones have already been imported). It was still functional though -- just had to keep selecting the ones I'd like to import.
Now it always gets stuck right after finding the photos, but before it pops up the box used to select images to import. Not sure exactly when this started happening, but I'm on 1903 with all the latest updates. I've tried Repairing and Resetting the Photos app, but it just hangs every time now.