I think your options will be extraordinarily scarce.
I assume you prefer to be in a riverfront town near the River for cruising and easy getting around. An option you might not have considered: Rhine hostels. The Bingen, Bacharach, Kaub and St. Goar hostels belong to the officiahl DJH/HI organizations. Don't shriek. These hostels have been remodeled and modernized in recent years to attract families and couples and offer an opportunity for groups like yours to stay together in hotel-room-like situations (private bath, doubles, triples, quads.)
The Kaub hostel is in an historic building near the waterfront and the Rheinsteig hiking trail. The others are located higher up and are a substantial climb from town.
This video consists of an interview with the Kaub hostel manager and provides a look at the interior of the hostel as well as a few of its guests. It's in German, but have a look. The two older male customers are former ferry captains who enjoy the hostel's view of the Rhine while they talk about old times. Obviously, hostels aren't strictly for young'ns anymore.
Kaub hostel reviews on Tripadvisor:
https://www.tripadvisor.co.uk/Hotel_Review-g1183932-d11672943-Reviews-Jugendherberge_Kaub-Kaub_Rhineland_Palatinate.html