100.000 missing photos migrating LR Classic Catalogue to Lightroom 6.5

Hi, I'm still using an old Late 2013 Macbook Pro with Big Sur (upgrading to a new laptop very soon), however my installed Lightroom Classic doens't work anymore since the Big Sur update. Then I downloaded LR 6.5 and agreed to migrate the catalogue. It said, that it had issues with 100.000 photos where the originals couldn't be found and that they couldn't be migrated. I'm confused by that since of course all the photos are on several external drives. I wanted LR to import my old folder structure with the links to the photos, it didn't import anything of that, obviously only the few photos that I had on the laptop directly about 2000. The old LR classic version doesn't open anymore under Big Sur. I do have a Time machine backup from before the Big Sur update. Help is very much appreciated!

Thanks a lot!!
Iris