What is correct in this situation?

Your answer is fully correct. In this situation, you must reduce your speed because you are approaching a priority road, indicated by the yellow diamond sign. The truck is already on this priority road, so it has the right of way. According to German traffic rules, vehicles on the priority road have preference, and vehicles joining or crossing the priority road must yield and adjust their speed accordingly. Therefore, you must slow down and let the truck pass before you continue.