What is the right course of action?

You answered correctly. In this situation, you are on the priority road, as shown by the yellow diamond sign. The red car is coming from your right and also wants to enter the priority road, so according to the “right before left” rule, you must let the red car go first, even though you are already on the main road. After the red car passes, you can go before the blue truck, because the truck must yield to you since you are on the priority road and coming from its right. Knowing how to interpret priority signs and the “right before left” rule is essential for correctly handling intersections in Germany.