Luggage transport problem as a tourist - Germany

Hi all,
I am planning to take a cruise trip over Rhine on my first day arriving Germany this time. The plan is, that I will arrive Frankfurt airport in the very morning, then take a train to Rüdesheim, then take the cable car up to the hill, then take the other cable car line to Assmannshausen. Following this is my trip cruise. I will probably visit St. Goar, Boppard. But in the end, I need to get to Braubach, as that is the place of the hotel I've booked. Then my trip next day will start from Marxsburg and then Koblenz. I will get a car in Koblenz, and then drive to Luxembourg.
However, my this plan seems to be hindered by my luggage case. It shall be quite a heavy case which does not make sense to carry it all around with me. Yes I could store it somewhere near Rüdesheim, but that means I will have to drive an additional circle back to Rüdesheim and pick it up. That will cost me 2 hours, if you consider the direct time between Koblenz and Luxembourg against the triangle I am going to make this way.
What I am think now is that, when arriving at Rüdesheim, I can lock it in the luggage locking box in the railway station. Then after I reaching Assmannshausen, I will try find some public transportation way back to Rüdesheim, and take it up to the K-D boat. In St. Goar, I can put it in the information center (and for free). Then I take it to Boppard, put it at the railway station there. Then I will leave it there, and go directly to Braubach for the night. The next day, when I get a car, I can drive back to Boppard and pick it up. Does that sound feasible? From Koblenz to Boppard is just 25 min, which is affordable to me.
Probably the pitfall here is that, I will need to find a public transportation from Assmannshausen to Rüdesheim. From google map, the distance is just 5.5 km. But are there any public transportation available and how much? Can anyone here give me some hint?
I would appreciate more if you have some better ideas, to solve the issue I have. Well, thank you in advance anyway, if you have patience to reach the end of this post, here.

I'm dizzy!
Try this service by the Deutsche Bahn, they will transport your luggage for you.

Yeah, there's no way I'm going to be able to read and process all that. But for all public transport info, including buses, go to the Deutsche Bahn website.