Rhine valley wine tasting suggestions

It is very easy to take the train from Frankfurt to Rüdesheim and then jump on one of the ships going to St. Goar. Take the train back to Frankfurt.
It looks like you have found the tour sold by ETS that gets really mixed reviews, but mainly bad about the meals and the wine. Not sure why they never improve these choices for their tours.

For wine tasting, you could do this either in St. Goar or in Rüdesheim.
One place that is fun is RheinWeinWelt in Rüdesheim. They have 6 tasting rooms set up in the old Asbach Factory and about 160 wines to taste? You buy a bag of coins and walk around and taste which ones you like, from expensive to cheap, dry to sweet, white, rose or red. All are local Rheingau wines.
https://www.rheinweinwelt.de/en/rheinweinwelten.html

I really question of the efficacy of a wine tasting at one winery. Unless you are planning to buy a few cases of wine to have shipped home, and want to pick a wine you want, why just taste wines from one winery. If you just want to taste and compare some different wines, go to a restaurant and order off the menu.

In 2008, I did a tasting of some wines at a winery in Cochem. He gave us two years of two different wines that they sold at the winery. I liked two, one of each variety, and didn't really like the other two. I kind of think the two not-so-good wines were provided to steer me to the wines they were selling. But I wasn't buying wine, anyway.

The next day I was in Bernkastel. I had lunch at a restaurant in town and sampled two wines, one a Riesling, and the other, I think, a Müller-Thurgau. They were both very good examples of their variety.

I think I got as much out of sampling those two wines at a restaurant as I did from the wine tasting.

The RheinWein Welt has wines from around 80 wineries in the Rheingau. Thus, making it one of the best places to do wine tasting.

Unlike the USA most wineries in Germany do not have their own tasting rooms. One winery I patronize has a small tasting room 20 km from the vines, on a side street in a small town. It's not a tourist destination. Towns like Rudesheim, or Cochem have multiple tasting rooms where you can find 40-50+ wines available for tasting and sales, but if you're idea of wine tasting is sitting on a terrace overlooking the Napa Valley while sipping a glass or two, those places are few and far between.

One place like that is Weingut Dr. Bürklin-Wolf, on the wine road, just south of Bad Durkheim.

Be sure to check for local, public wine festivals, including closed streets in Frankfurt, that time of year.

Not on the Rhine, but we went to the very local Thiesen Winery in Senheim. It’s very small. We bought a bottle of wine and were sitting outside drinking it when a Belgian sat down with another different bottle and we tasted it. After the winery closed the owner came out with yet another bottle. We all sat around and chatted the best we could for about an hour. Memories.

Thank you everyone for all the suggestions! The RheinWein Welt seems like a great place and will definitely look more into it.

October is too late for larger wine festivals - more in summer. Local ones possible.

I suggest to look for wine tasting in Rheingau region between Wiesbaden and Rüdesheim.
This pocket guide contains dates of wine tasting stands and Vinotheken.