There is one question left. How do I build the checked out local and updated Plone docs repo, as I think that the make process pulls the docs from the official Github repo, where my changes are not yet published?
As a quickfix I copy/pasted my local changes into the checkouts directory () -but there must be a better way!
P.S.:
With these many dependencies this setup looks like the first real world use case I encounter for possibly using a Docker image?
as for the Docker question; there is work on that going on for future versions. Long story short: for the current (imperfect) Papyrus setup, we are not likely to change the process. Limited resources and stuff...
During lunch break I came up with a solution. I do not checkout plone/documentation manually, but via papyrus and apply the changes directly in the checkouts/documentation subfolder on a new branch, and then create the pr from that branch.