I have done this route many times with different friends and family members.

The best part is between Rudesheim/Bingen to Rhens.

But if you go to Rhens then you might as well go to Koblenz. I always suggest going downstream (towards Koblenz) because it is significantly quicker. And in the end you have enough time to view the scenario and 4 hours on a boat is long enough (Rudesheim -> Koblenz).

Bear in mind if it is a weekend in Summer the boat will probably be very busy! So to be sure to grab a seat outside you need to get on in Rudesheim. And if you are not on early you might not be at the edge. But this varies depending on which boat it is.

I would also suggest spending some time in Koblenz, it is a nice city and you can visit the "Deutsches Eck" (Meeting of the Rhein and Mosellle rivers) and Castle Ehrenbreitstein (reachable via cable car from close to when the boat trips finish. This you can do in the afternoon after your boat trip, and then head back.

Then you can either spend the night in Rudesheim or Frankfurt. I would suggest Rudesheim if you want to mix with the tourists, which will be mostly German. Or head back to Frankfurt for your meeting the next day.

I would suggest it only makes sense to stay in Mainz if you intend to spend some time looking around the city.