If you're doing a LR deep cleaning, consider splitting up the task into manageable chunks.
I started using LR without a good plan, and eventually I had 65,000 entries in the catalog. There were a lot of duplicates and different edit versions. Eventually I decided I needed to clean it up.
The first thing I did was to generate a set of smart collections based on year of capture. I then exported each as a catalog for each year. That broke the task up into about 15 catalogs, some large, some small, but all a much more reasonable size than 65K images.
Then I went to work. I looked at each image, tried to do some triage to delete the junk (I have a hard time throwing things away). I got a duplicate finder to try to locate duplicates and deleted the extraneous ones (see
https://www.lightroom-plugins.com/DupesIndex.php). After paring down the number of catalog entries, I went through and added appropriate keywords where needed.
The process was fairly quick for the early years. I could go through a years worth of images in one free-time period (usually an evening). Later years, when I took more photos, took several sessions. LR provides a number in the Library Grid View so I would just write the number where I stopped on a sticky note and stuck it on the monitor. Going through the photopile took me a couple months of free time, but I got the pile down to about 15,000 images.
When I encountered an image that was scanned in, I placed that in a separate collection. When I got through all the years I took the first year, exported that catalog as "Temporary Catalog 2011" (I think that was the year I did this). I then imported the next year's catalog, then the next, etc. so I eventually had all the important images (actually the images I wanted to keep, not all of which were important) in one catalog. I then took the "scanned Images" collection and exported it to another temporary catalog and deleted those images from the temporary catalog (they were all still in the yearly catalogs so this was a nondestructive delete). I then went through the scanned images and tried to change the metadata to reflect the actual capture date (or best estimate thereof), and took the result and re-imported it into the temporary catalog, which was then re-named "Master Catalog 2011".
For the most part, when deleting junk, I deleted it from the disk. There were occasions where I thought the junk might have some redeeming value so I left it on the disk but deleted it from the catalog.
When I got through this process, I backed up the resulting files and catalogs (including the temporary and yearly catalogs) thoroughly (3 copies in different locations plus the cloud).
PS: I now export the master catalog at the beginning of each year, with a new year in the file name. Part of my CDO (similar to OCD but the letters are in alphabetical order, as they should be).