IMHO your biggest mistake is having more than one catalog. Also you don't have to start a new catalog just because you moved your images from one drive to another. LR has a feature to Relocate missing files and folders.
Sorry I have no idea how to merge catalogs to just regain the keywording you did.
It sounds like you're creating a complicated mess, and I recommend that you restart the merge process from scratch. Otherwise, you could be facing days of madness and data loss.
Go to each of the old catalogs and reconnect the photos with their new locations: Adobe Lightroom - Find moved or missing files and folders
Then create a brand new main catalog. In that new catalog, do File > Import From Another Catalog for each of your old catalogs. Set the option File Handling: Add New Photos To Catalog Without Moving. That will bring in all of your keywords, captions, labels, collections, stacks, etc.
I kind-of agree with John's advice, though I'd start by freezing things - don't let the mess get any messier. Back up every single catalogue, and consider doing a complete backup of your photos.
You mention "importing a second (the latest) catalog". Does this record all the 500k images with all their keywords, but in the old - now wrong - locations? If so, you can start with this catalogue, treating it as your master catalogue and fixing all the missing folders in Folders panel. You should not then need to do any Import from Catalog operations. If not, but it is the best of the catalogues, then fix the folder locations and then proceed as John suggests.
In your master catalogue - and you will be best served by having one master catalogue - keep running the Find Missing Photos command until it reports 0 missing photos.