https://feedback.photoshop.com/Ok. Now we are getting somewhere. Not a perfect solution by any stretch of the imagination but I can import......kind of.
Everything is so random. I've had some success too with just killing Lightroom. Moving all the files I want to import/move into a new folder and then back in to the original folder and then attempting the import a 2nd time. It seems to work especially well when I only try to move photos into the folder and import those and then move video files into the folder and import those. Like it only wants to show previews for one type of file at a time or something. Go figure. I imported 400+ photos and then a few hundred videos this way tonight and didn't have to kill any processes in Activity Monitor first. Whatever.