What is the right course of action now?

Your answer is correct because both reducing speed and remaining in the left-hand lane are appropriate actions in this situation according to German traffic rules. When encountering slower vehicles ahead on a multi-lane highway, you should adapt your speed to maintain a safe following distance rather than overtaking on the right, which is generally prohibited. Staying in the left-hand lane (the proper overtaking lane) without attempting illegal maneuvers ensures safety and compliance with traffic laws. Continuing to drive as before or trying to overtake on the right would be dangerous and against the rules. Thus, slowing down and staying in the correct lane is the right course of action.