boppard

I would suggest you take the train to Bingen or Rüdesheim and catch a boat back from there, to avoid the slow sailing up stream.

KD is not very forthcoming on ticket prices on their website. There is a day pass available from viator.
https://www.viator.com/tours/Mainz/KD-Rhine-Pass-from-Mainz/d28883-3320KDRHINEMAINZ?subPageType=important-info

Of course if you want to take your time, going upstream is slower, and that could be a good thing if you want more time to gaze at the scenery. The boats may also may be a bit less crowded. We did this a few years ago, going downstream from Mainz, and noticed a large number of tour buses at Ruedesheim and a throng of people boarded there, then got off at St Goar, where the tour buses had repositioned themselves. During that part of the cruise, the description of the sights given in on the PA system was in about 10 languages, including Chinese, Korean, Japanese, Arabic, Italian, French, Spanish, English, Russian, and yes, even German.

At any time you feel you've had enough, you can always get off at a town on the west bank, and take a train back to Boppard.

Take the train to Bingen and get on the boat ( show your train ticket and get a discount ). Go all the way to Boppard. If you want to visit Bacharach, St. Goar or Oberwesel, make a separate train trip.

A KD Rhine Pass is $39.14, which is about 33€ at today's exchange rate.

IF you take the train to Bingen and come back to Boppard on the KD boat, the full fare is 27,20€ but you get a 20% discount by showing them your Bahn ticket, so it will only cost less than 22€. You will save money vs the pass. Even if you do the trip round trip on the boat, for 30,20€, with no Bahn ticket discount, it will be less than the pass.

With a KD ticket from Bingen to Boppard, you can get off the boat at intermediate stops, like Bacharach, and rejoin the boat later, as long as you inform the crew as to what you are doing.

You can find schedules and fares for the KD boats here.