I think Tim is right about Burg Eltz - and an excellent alternative is located across the river from Boppard - Marksburg Castle in the town of Braubach. Although we've discouraged cruising north of Boppard, the KD cruise line has an 11:50 boat from Boppard that gets you there in just 30 minutes and right in time to walk up to the castle take the 1:00 pm ENGLISH tour (you can also get there by train or by ferry + train, and cheaply, or for free, with your Guest ticket.)
Please remember that the advice you get HERE is individualized based on your comments and also subjective - it is not necessarily what you read in Rick's guidebook.
Rick began publishing back in the early 80's and "came out shooting from the hip." Since then he has familiarized himself a little more thoroughly with the Europe he wrote about. 30+ years after his first travel book, after decades of praise for Bacharach, he took his first steps in the neighbor-town of Oberwesel - a town already well recognized in many other travel publications as a fine town to visit. You can read what he blogged about Oberwesel 6 years ago, after his first look around:
https://blog.ricksteves.com/blog/visiting-the-rhine-river-valley/
He writes that Oberwesel has "the best collection of historic Rhine artifacts... within the romantic Rhine gorge."
Another discovery new to Rick in Oberwesel: https://blog.ricksteves.com/blog/the-wine-queens-or-wine-witches-of/
My basic point: Rick's relationship with and strong recommendations for Bacharach over other Rhine towns were not necessarily informed by visits to other nearby towns with which he might compare Bacharach.
In this travel article on the Rhine, Rick discusses cruising...
"A Rhine boat cruise is the most relaxing way of all, but it's slow. I'd do just the hour-or-two span from Bacharach to either St. Goar (nine miles) or Boppard (17 miles) by boat, and the rest by train."
Well, the general consensus on this forum says differently... it's a measly 40-minute cruise from Bacharach to St. Goar (€16.) If you begin the cruise in BINGEN, back in the area where the Mäuseturm, Burg Ehrenfels, and Rheinstein Castle are located, you'll spend €5.40 more, but you'll have a full 1.5 hours on the cruise to St. Goar - and of course you won't miss out on these and many other nice sights on your way to Bacharach and St. Goar (or Boppard.)
I suspect if Rick were to board the cruise in Bingen sometime, and visit / stay in some other Rhine towns, then someday he might provide somewhat different advice to his readers. But it looks like Rick's audience will continue to be sent to Bacharach for the time being.