What is the right course of action now?

You answered correctly by choosing to stay in the middle lane. In Germany, the basic rule on highways with multiple lanes is the "Rechtsfahrgebot," or keep-right rule. This means you should drive in the rightmost lane whenever it is safe and practical to do so.

If you are overtaking a slower vehicle, you use the left lane. Once the overtaking maneuver is complete and the right lane is clear, you should return to the right lane to allow faster vehicles to pass on the left. However, if you are not currently overtaking or if the right lane is not clear and safe to enter, then staying in the middle lane is correct to maintain traffic flow and safety.

In this specific situation, staying in the middle lane allows you to keep moving safely without forcing unnecessary lane changes that could confuse other drivers or cause danger. Always make sure to signal before changing lanes, check your mirrors and blind spots, and only return to the right lane when it is safe.

By following these principles, you comply with German traffic regulations, promote smooth traffic flow, and maintain safety on highways.