5 days out of Frankfurt

I stayed in the Black Forest area for 4 nights (in Gengenbach) and really enjoyed it. The small towns along the Black Forest Railroad were charming, the scenery was very pretty and I loved seeing the Vogtsbauernhof open air museum. So I found it worth it to go there. OTOH, I had no inclination to go to Baden-Baden - it sounds very touristy and just wasn't for me. YMMV, however.

That said, I would suggest just staying in Bacharach for your whole trip. You only have 4 nights (plus your last airport night), which means you really only have 3 full days for sightseeing. Your first day will be spent getting your car, then driving, then checking in and then maybe time for a quick walk around town, dinner and crashing for the night with jet lag. The next day will be your only full day in the Rhine area before you leave for Baden-Baden the following day. The Rhine is a beautiful area and if you are interested in all the things you mentioned above, then I would again suggest staying there for the four nights, so that you have the chance to leisurely visit wineries, the towns and take a river cruise.

It would be helpful to know the month of the year. A few things might close in August vacation month, like a famous restaurant or a winery. In September or October, there would be wine festivals to visit. In my experience, serious wineries in Germany require reservations. High-end hotels will make them for you, but I imagine it's better to do so in advance in high season.

(Edit: corrected) Is this whole trip to Germany from Seattle 5 nights long? These are really short stops, with hotel-change overhead. However, 2 nights in Bacharach is fine. Remember that you are using the local train with a boat ride, not the car. Use the search box here for "middle rhine KD". Do you want to enter some castles, or just ride the boat and visit wineries? What is your objective in the BF; I mean "hiking", "waterfalls", hotel spas? Would you consider driving back towards Frankfurt rather than south from Baden Baden? Do you strongly prefer one type of wine over another?

It doesn't directly apply to your route, but you might want to read part of my trip report,
https://community.ricksteves.com/travel-forum/trip-reports/baltic-germany-fachwerke-landesgartenschau

It is not true that the Mosel is "like" the BF, but would you consider a closer-in second few days? I suppose you have bought the air tickets, but I might try to fly home from Basel or Zurich on your general itinerary. (For example, I believe you could return the car in Weil am Rhein (i.e. in Germany) and call a Swiss taxi to the Basel airport (or a Basel museum.))

I'd recommend the villages of Rudesheim and Oestrich-Winkel especially for great Rhine wineries. Better yet, stay there, so you don't have to drive, unless there was something else you were really counting on seeing in/near Bacharach. Don't drive if you've been drinking wine. I'd also recommend venturing down the Nahe as there are top notch wines to be found there. If you don't care about visiting actual wineries, there will be lots of wine taverns to visit in the area, where you can sample a wide range of producers/styles.

Timing does matter, as harvest usually starts in September and runs into October or later, depending on the year. Smaller wineries will not be able to accommodate visits during harvest.

Sorry, forgot to mention wineries to check out. Georg Breuer & Leitz in Rudesheim, Künstler in Hochheim, Spreitzer and Schloss Vollrads in Oestrich-Winkel, and Goldatzel in Johannisberg. Also Kruger-Rumpf near Bingen, if you venture down the Nahe. Most have tasting rooms open to the public, but you may want to contact them for hours and an appointment, just to confirm.

Thank you, all! This will be the start of a month long trip (istanbul, Greece next). We are stuck with going in/out of Frankfurt. I'm not a fan of hotel hopping so sounds like staying in Bacharach all nights might work best. We will be there last week of September.
We like all wines! Other than the river cruise, we thought we would drive around, to explore.....unless the advise is to take the train from town to town? Thank you!

Besides the chance of wine festivals (which can provide really informal, easy tastings in a town square), you have a chance for never-exported wines. This includes non-"fine" wines, I mean like Neue Susser, which is too low-alcohol to be bottled and shipped. It's just an interesting side-light on the wine tradition/business.

I recall some bottled things distantly related to Neue Susser, at the monthly Duesseldorf "Fish Market" festival, but the vendors warned me (in German) that the bottle could explode in the airplane hold because the whole product was not intended to be stored for any significant time.

If you are doing wine tastings, then you shouldn't be driving. Take the train along the Rhein. In fact, if you are landing in Germany after a trans-Atlantic flight, you should not be driving a car at all after landing. This is so dangerous.

One of my favorite places to take people is in Rüdesheim at the former Asbach factory across from the train station. It is called RheinWeinWelt and it is the easiest place to do wine tastings. https://www.rheinweinwelt.de/en/rheinweinwelten.html

2nd favorite place is right outside of Eltville at Eberbach Monastery. They have almost a 1000 years of history producing wines and the basilica there is fantastic. (Eltville is charming and quaint too. )They produce some of the top wines for the Rheingau area. https://kloster-eberbach.de/de/wein

Do you want to see anything in Frankfurt? Not really sure what you want to see in the Black Forest and Baden Baden? Do you want to go to spas? They are everywhere and you don't have to drive that far.

"I'm not a fan of hotel hopping so sounds like staying in Bacharach all nights might work best. We will be there last week of September... We like all wines! Other than the river cruise, we thought we would drive around, to explore.....unless the advise is to take the train from town to town? Thank you!"

Exploring near Bacharach:

1) Last week of September: The annual Boppard Wine festival is your passport to wine fun. From Sept. 23-26 this Rhine town, roughly 20 train minutes north of Bacharach, will be set up with wine and food stands from multiple local wineries. Visitors stream in from all over for this event, which actually takes place over two weekends (the 1st October weekend as well.) The busiest day is Saturday, when the fireworks and the evening party cruises happen. There's live music in town in the evenings as well. The guide to Boppard below will inform you on things to see/do.

Boppard's Weinfest on market square
Guide to Boppard

Getting to Boppard: direct trains from Bacharach run hourly or better all day long. Only a naive international tourist would think it's wise to take a car into a small town like Boppard during a Weinfest. This group from Duisburg-Huckingen got to the Boppard Weinfest by train, and you should probably do the same:

https://huckingen.de/wp-content/uploads/2018/09/Boppard-9.jpg
https://huckingen.de/wp-content/uploads/2018/09/Boppard-1.jpg

2) Oberwesel: Another handsome old town with in-town wineries, this one only 5 minutes by train north of Bacharach. Walk the old town wall for sure.

3) Rheinfels Castle in St. Goar: 10 min. by train from Bacharach. Shuttle available from in town to the cliffside castle, or walk up.
https://www.tripadvisor.com/Attraction_Review-g198631-d268086-Reviews-Rheinfels_Castle-Sankt_Goar_Rhineland_Palatinate.html

4) Rhine Cruise: Don't board in Bacharach unless you wish to miss out on half the scenery. The best strategy is to take the train 20 minutes south to Bingen (Rhein Stadt station) and board there for a cruise to St. Goar or Boppard - then return by train. This cruise covers the most scenic segment of river.