Same problem since July: can't install 21H2 neither manually via cumulative update nor via windows update. Tried all possible troubleshooting methods:
dism, sfc, scandisk, delete windows update repository, uninstalling edge and a few more I don't remember.
Download the media creation tool. This is the only one that worked for me. I tried everything you did to no avail but the MEDIA CREATION TOOL did its work. Hope this helps you too.
Here's the link: https://www.microsoft.com/en-ca/software-download/windows10
Click on Download Tool Now button
And do what with the media creation tool? When you say "install it" how far are you going through the process in the app?
Just follow the instructions. It will download necessary updates for you. Please make sure it says keep personal files and apps. It took a long time for me but it did its job.