Well, I see now why didn't anyone reply, this is a tough issue. :)
Solved by trying to install newest updates, checking CBS.log for errors and then sourcing corrupted files from a LOT of msu's. :) Also had to download 1903 and 1909 images (and convert ESD to WIM). Overall, had an interesting ~10 hour adventure. :)
Used SFCFix and experience from other posts. :) So thank you all for that!
When all the updates installed, tried upgrading to 20H2 again and SUCCESS. :)