What is the right course of action?

In this situation, the correct response is to continue slowing down and wait before the obstruction. When you see a slow-moving vehicle ahead and there is an obstruction, especially in bad weather and with wet, slippery roads, German traffic rules require you to adapt your speed and be ready to stop if needed. Overtaking or switching lanes immediately is not allowed unless it is safe and clearly permitted, which is not the case here due to the weather, road, and traffic conditions. It is important to always maintain a safe distance, reduce your speed, and only proceed or overtake if you have a clear and safe opportunity. Your selections were correct.