> One of the most difficult aspects of securing it was that the required documentation was a moving target; online sources conflicted with one another and each reviewer of your application seemingly applied their own new criteria as well. As a result, one of the most successful strategies was insistence; arguing with your reviewer and demonstrating how you did in fact have sufficient documentation and that they were wrong.
I had this exact same experience when renewing my contractor visa in Germany. I'd suggest bringing a printed copy of the requirements list from the German ausländerbehörde website with you whenever you show up to these appointments.