Weekdays in Cochem will undoubtedly be much better, as perhaps, is the time of year you are going. The first two weeks of October, we learned, are when a lot of German people take vacations. First there is German Unity Day, on October 3, which this year fell on a Thursday, and a lot of workers get the bridge day for a long weekend. Also, they have school holidays in early October. And maybe because the tourist season is supposed to be over, that is when German people get out to see Germany? And on a nice day after a terrible summer, watch out.
We also noticed quite a few of what I would call weekend party boats on the Rhein and Mosel. That's one of the things that hits Cochem, I bet. For a lot of reasons I am going to take better notice of how I plan to spend my weekends.
Cochem is the only place where we found crowds to be a bother. On the Rhein it was fine. Of course, the weather was really marginal, though it didn't stop us. We stayed in Boppard at the Bellevue Rheinhotel. Recommendations from the forum convinced me to stay there. It was good middle location, and more lively than the smaller towns. Bacharach is absolutely adorable and we enjoyed our day there, but it was pretty deserted. We took a spur of the moment trip to Koblenz and enjoyed the Deutsches Eck, the Seilbahn ride across the river, and walking back along the riverfront towards the train station. Then we went to Braubach (adorable but also really tiny) and then to Marburg castle. We bought tickets for the 4 p.m. tour in English and ended up being the only people on it. So we enjoyed an extra-long private tour given by a very knowledgeable young man. It was a wonderful tour and we learned a lot about castles in the middle ages. I wonder if that late afternoon slot works other times or places.
Our experience with Rüdesheim: We decided to continue down the east side of the Rhein to see Rüdesheim, but never got there. We trained down, but it's a hike from the train station to the Drösselgasse, and to get back to Boppard, we would have to take a ferry. With the lateness of the hour we got cold feet. I think there is a ferry that goes train station to train station across the river, but that one was done, so we went straight across and ended up having to walk about an hour to the Bingen train station to get back to Boppard.
You have gotten a lot of really good advice here! It really is all in the details and what you like, but you have time to figure it out.
I have been to Germany several times and would love to hear about your 35 days!