The solution at the end almost looks like the manual setup of terragrunt which we are using to manage lots of base infra in many different accounts.

What would be interesting here would be to see how they actually reference the outputs from one layer onto the next layer. That is something that is not even solved nicely in terragrunt and one of the major annoyances for me there. Using dependencies and the mock_output option is creating lots of noise in the plan outputs as the dependencies are only completely resolved when terragrund applies all the modules.

But it seems I also missed a few additions to terraform - so probably there are better ways to take outputs from one terraform run into another one.