In the Azure DevOps world, moving from one tenant to another is not a very familiar concept based on Microsoft documentation. I had to do exactly this. The Azure DevOps Migration Tools did the trick for most of the ADO Boards-related things.
Repos were copied over by pushing to a new Git source.
Pipelines are stored in the Git repository as YAML and were imported / linked again.
One large task was migrating the pipeline library. You can’t export the values secrets in the library, but I used a tool to make sure to copy everything over. Having the secret variables copied over with empty values was already very valuable to me.