What is the right course of action?

You chose correctly. When approaching slower vehicles on the Autobahn, you should always reduce your speed if the traffic situation changes, for example, if you are approaching congested traffic or if another vehicle is merging into your lane. Reducing speed increases safety and gives you more time to react to potential hazards. Continuing at the same speed or moving lanes without a clear need could be dangerous. In German traffic rules, adapting your speed appropriately is more important than simply maintaining speed or changing lanes, especially when situations become unclear or could change suddenly. This rule helps ensure you have enough time and distance to react safely and avoid accidents.