Romantic Rhine Itinerary

That sounds like a wonderful day! And you included some of the most interesting castles - both very different from each other.

July can be very hot in the Rhine valley. Especially if you are biking in the sun. You may want to build in some time to take a shower and change before heading out for dinner ;-)

Shopping in the evening could be an issue. Bacharach is very touristy so you might be lucky. But usually only in large cities would you find stores open beyond 6.30 pm. Except maybe for one day a week like Thursday night. If shopping is important to you I'd inquire about opening hours.

The schedule seem doable, IF everything falls into place. You seem to have almost fit your schedule with the Rhein K-D cruises, but there are a few differences. Make sure you note them, you don't want to "miss the boat". The schedules are on the K-D website.

It's 8.7 miles from Bacharach to St. Goar. That should be no problem in an hour if traffic doesn't cause a problem on the narrow riverside road. The last km or so is up a steep hill.
I spent almost three hours exploring Rheinfels, but maybe I was more interested. However, you could eat on the boat. I think there was food service, but you could always bring a picnic.
The boat schedule says the boat leaves St. Goar for Braubach at 1155 (not 12 Noon). The boat arrives at Rheinfels at 120 PM, but that is at the riverside. The castle is at the top of a hill. If you cycle to the top it will take a few minutes. There will probably be a trolly to meet the boat and take you to the top. That takes time too.
You can only tour the Marksburg with a guided tour. Hopefully one will be leaving shortly after you arrive. I'm not sure 1-3/4 hours, waterfront to waterfront is enough for Marksburg.
The K-D boat back up the river leaves Braubach at 305 PM and gets to St. Goar at 520.

An alternative that would give you more time in Braubach would be to take the 451 PM train from Braubach to St. Goarshausen at 513 PM. Then you could ferry across the river and ride to Bacharach.

Lee and Beatrix, thank you both very much for the advice! Beatrix, I'll probably be doing mostly window shopping as I need to save my wallet for Rothenburg and the delightful Christmas shops, but I will be mindful of the early closing hours! Being from Vegas I'm not used to things closing before 7PM or at all, really.

Lee, thank you very much for the train suggestion! I was a bit worried about not spending enough time in Marksburg and Braubach. Did you get that train schedule from the DB website? Also, would the train allow me to carry my bike on board?

It's a doable day. It's an almost flat 14km ride between St. Goar and Bacharach but uphill for the last bit to Rheinfels. I can't recall the wind ever being an issue for biking.

I like more time at Rheinfels because there is a lot to explore but it depends on you. Arriving when it opens allows you to explore before most people show up. Some people will do more or less depending on how extensively they like to explore (do you want to duck walk through the mine tunnels?).

When I've been there (only September or October), there isn't much shopping in Bacharach after dinner. Dinner options also thin out quickly if you wait too late. But it is still great for an evening stroll. It may be different in high season.

I should have added that one time we missed a K-D boat. We had a ticket and were there on time. The boat that pulled in was packed and they only let about ten people from the dock board the boat.

This was in early October. There were about a hundred people waiting to board. We had to return our tickets (which they did) and change our plans.

K-D boats run pretty reliably but you have no control over the numbers of people on the boat. Like here, there are regulations on how many people a boat is allowed to carry.