You want to turn left at the next junction. What is the right course of action now?

You answered correctly. When you want to turn left at the next junction but there are stationary vehicles ahead, you must do two things: First, you remain behind those vehicles until it's safe to proceed, as overtaking stationary vehicles at an intersection is generally not allowed and could be dangerous. Second, you must indicate your intention by using your left turn signal so other road users know your plan in advance. These actions ensure both your safety and clear communication with other drivers and pedestrians. This is the correct and required behavior according to German traffic rules.