OK, some of the mystery seems to be explained. You've obviously updated to LR Classic at some recent point, and when you do a version upgrade such as that Lightroom will create a new version of the existing catalog.....so it looks as though your LR6 catalog was Lightroom Catalog-2.lrcat, so the upgraded version would be the Lightroom Catalog-2-2.lrcat catalog (which is why LR6 cannot read that, as it is "too new"). Crucially, the catalog upgrade process also "steals" the existing previews cache....thus the Lightroom Catalog-2 Previews.lrdata would simply have been renamed as Lightroom Catalog-2-2 Previews.lrdata.
When you then abandoned LR Classic and reverted to LR6 and opened the Lightroom Catalog-2.lrcat, because the existing previews cache no longer existed (because it had been renamed by the upgrade process) a new empty previews cache is created. That then would normally start to populate the blank thumbnails with new previews, but that requires the actual images to be online....if they are not, the thumbnails remain blank.
To proceed to the next stage, you have a choice:
1. The license type as per your System Info is actually a Creative Cloud license, which should allow you to install and run the latest version of Lightroom, which is Classic 8.0. If you want to go that route, you would install Classic using the Creative Cloud desktop app (which you may need to download and install first, if not already installed). Then you would open that Lightroom Catalog-2-2.lrcat catalog, and that will use the corresponding previews cache, so you would then see all the previews in the Grid, even when the images are offline.
2. Or, if you prefer to stay with LR6, rather than upgrade to Classic, with LR closed you could first delete the existing "Lightroom Catalog-2 Previews.lrdata" package, then rename the Lightroom Catalog-2-2 Previews.lrdata package to bring it back in line with the LR6 catalog, i.e. rename it to "Lightroom Catalog-2 Previews.lrdata". Then, using LR6, open the Lightroom Catalog-2.lrcat, and you should then see the previews in the grid.
Having done either of the above (please take care to ensure you don't get the "-2" and "-2-2" files mixed up), you should then bring ALL the drives online, and then run the Library>Find All Missing Photos command to see how many missing images you still have.