What must you be aware of here?

You answered correctly. In Germany, when approaching a roundabout, you must continue to brake and reduce your speed to be able to yield if necessary. There is always a "Yield" sign (Vorfahrt gewähren) at the entrance to a roundabout, which means you have to give the car already in the roundabout right of way. It is not allowed—and not necessary—to indicate when entering the roundabout; you only use your turn signal when you intend to exit the roundabout. By selecting to continue braking and to give right of way to the car inside the roundabout, you followed the correct procedures according to German traffic rules.