Moving a catalog and photos between computers

I'm using standalone Lightroom 6.14 but there doesn't seem to be a forum for that. The situation is so complicated I can barely describe it. My old Macbook died, terminally, so I had to buy a new one. It's a model with a small SSD, so I also bought a new external disk as there was no way 500GB of photos would fit on the SSD. Getting the photos off the Time Machine backup of the old computer was a challenge, but I finally managed it. So now I have a folder called Pictures on the new external disk. Inside it is the structure I had in Lightroom on the old computer. I always import into a subfolder with the name of the place where I took the photos. So for example a folder for Paris would be Pictures/Paris. Inside Lightroom classifies the photos by date, with a folder for each year and day folders within that. So Paris/2019/2019-01-31 for example.

I also copied the catalog and understood that I would need to open the catalog and use Find Missing Folder to relink to the new location of the photos. When I open the catalog though, and look at folders in Library view, all the top level folders are absent -- even though I can see them in Finder -- and only the year folders show. So there are dozens of folders called 2014, 2015 etc. That makes it impossible to relink them because apart from there being hundreds of them to relink individually, I can't tell which place each belongs to. See screenshot below. Right clicking on Macintosh HD doesn't offer a Find Missing Folder option because it's not missing -- but the folders aren't on that disk, they are on the external drive. I can't just import the photos into a new catalog because I'll lose all the metadata and editing for thousands of photos. Help!!