Thank you both for your informative replies. We look forward to the experience, which I'm sure will be enjoyable. We prefer to go independently, too, and it appears the ship docks are convenient to town at each stop.
We lived in the Mannheim area decades ago, and have visited H'berg and the castle many times. However, those days we drove over and avoided the public transport rigamarole. This trip we are being accompanied by DW's sister and niece, who've never been there before. The boat docks somewhat behind the Mannheim Hauptbahnhof, so catching a train to H'berg is not a pro