You should find this cruise very enjoyable, you will be against the river flow, this is the better option as the boat will be moving slower. It is very interesting when passing through towns although in between these the scenery can be rather boring. The best stretch is from Koblenz to Rudesheim which is 40 miles of fascinating and very beautiful scenery with many castles and pretty villages, so in this stretch be prepared to dash from one side of the boat to the other in order not to miss anything.
Personally, I wouldn't do this trip again but use the KD line which is similar to a river bus between Rudesheim and Koblenz, stops at every town and village and is very cheap, the real way to see the Rhine.

Wife and I made the trip last year in July. The Rhine and Danube rivers were low. Had to change to buses twice and were on three different Viking boats. Amsterdam to Budapest. Some photography had to be made through the bus windows with reflections. Polarizing filters not useable due to speed of buses and lack of time to focus. On the river much of our photography was from the starboard side, simply because our cabin was on that side. Picture opportunities about equal from both sides of ship and many photos of other passengers and crew. If I took the trip again would go in April or May. Weather good at that time of year and water is higher. Temperatures in July were in the 90's F. and quite warm for walking tours. Viking crews were great and most helpful. Low river was not their fault and they really tried to make up for it.

The Mrs and I did the comparable German ship Rhine tour, frequently docking alongside the Viking ships. Both have upper decks which are great for viewing both sides of the river, especially during the Castle stretch between Rudesheim and Koblenz. I found the 12-60mm on a Lumix GX85 to be sufficient to my needs, plus a 20mm F1.7 for low light situations.

Basel has many museums of note, more than you'll have time for. Speyer Dom is the largest Romanesque church in the world, do visit the crypt with the Emperors buried below. We arrived in Rudesheim after the stores had closed but the "shopping ladies" calmed down after a visit to one of the many Wein Stubes to try the local vintages. The castles come one after another, take your pick. Most have been restored and painted in a common decor. Koblenz is lovely, nice view from the Ehrenbreitstein fortress across the river and worthwhile in itself. Our boat traveled during the night to Cologne, leaving us with a 2 hr early tour before departure. Since the tour returned to Cologne after Amsterdam, the short tour left one with a hint of what the city has that you could visit at the end after the trip. Of course the Dom both inside and climb the tower if you're energetic, possibly the Roman Museum next to the Dom, one of the many restaurants if only for the local Koelsch Bier that is served in a 7 oz glass - order two, maybe the Chocolate Museum on the Rhein in a ship. North from there the land is flat with fewer sites so enjoy the shipping views.

Learning a few German words like "Please, thank you " and "where is the bathroom" will make points with the locals.
Rhein

Like quite a few other Hoggers, I have done this cruise with my wife, twice. Wonderfully relaxing and interesting . The cruising stretch near Koblenz is great , castle after castle on the riverbanks , used a 70-200 most of the time . Around the towns etc found 24-70 fine . I could have done with something a bit more versatile as some other have used. Would have been easier.

Cologne is unreal . Research the construction of the Cathedral if you get a chance . Unbelievable history and story but here is a boring Cologne story for you .

My wife and I went ashore at Cologne and did our own thing, having previously been on the organised walks etc. We went in a small hotel for a coffee and the waitress who served us started talking to us. She wondered why we were only in Cologne for just one day . Told her we were on a cruise. Who with ? Viking ? Which boat ? Helvetia . My boyfriend works on that boat! We met when working together on a cruise ship. She then said that she rarely sees him because of his work and was surprised he was in Cologne as she had not heard from him. Straight back to the ship and down to the kitchens . Asked for the guy by name . He came out and got a real shock when we told him a message from his girlfriend. His co-workers were all around him and rolling about laughing, he had finally been caught out .

Facing the Cathedral is a large square which looks down slightly on it. In the square is a famous coffee shop with a large open air area. They serve wonderful pastries and drinks and it "smells" of the past opulence and style of Germany . In particular go downstairs and use the restrooms . I am not going to say any more and leave you to get the surprise we did.

Enjoy your trip.