Overnight or Half-Day Stop Between Rhine and Bavarian Alps - To Break Up the Train Ride?

I am no "train travel" expert, but if you travel south via Würzburg you will pass through the Franken wine region. May take some time and bus trips (overnight), but you could see small wine towns like Prichsenstadt, etc. that still have most of their wall/moat (Stadtmauer) and many timber frame (Fachwerk) buildings. You would catch Spargelzeit (white asparagus) time too.
https://www.prichsenstadt.de/
To the west of Würzburg is the small town of Miltenberg. I see it discussed in this forum from time to time.
https://www.miltenberg.info/
Farther out of your way is the Fränkischen Schweiz. Towns like Pottenstein. This area would probably require a car and more time.
https://pottenstein.de/startseite-tourismus
Landshut has unique architecture and I would not miss it, but most of the Fachwerk is covered-up with Putz (plaster).

Saw this on the web: http://www.fachwerkfreunde.de/

"...will we see sufficient fachwerk in towns along the Rhine?"

The "Fachwerkstrasse"-associated towns invest in a membership to be included but they are certainly not the only examples of Fachwerk towns - many Fachwerk towns are just not affiliates - and you will find many Fachwerk buildings on the Rhine - just depends on which towns you visit there and on the Mosel.

Some commonly-visited towns:

Braubach (Home of Marksburg Castle)

https://www.romantischer-rhein.de/uploads/pics/Altstadt_02.jpg
http://djtravel.homestead.com/files/braubach_-_hotel_1610.jpg

Bacharach
https://upload.wikimedia.org/wikipedia/commons/4/4c/Bacharach_BW_9.JPG
https://www.fotocommunity.de/photo/renaissance-fachwerk-bacharachmittelrhein-hozi/38666089

Linz
https://www.rhein-eifel.tv/fileadmin/_processed_/e/0/csm_linz-rhein-gasse-mit-fachwerk_81e6cf119c.jpg

Oberwesel
https://mittelrheingold.de/wp-content/uploads/2018/07/marktplatz-oberwesel-cr-rrt-henry-tornow.jpg

Fachwerk on the MOSEL...

Cochem
https://www.rheinwanderer.de/wp-content/uploads/2018/10/22_Marktplatz-der-Altstadt-von-Cochem_1000x667.jpg

Winningen
https://farm5.staticflickr.com/4120/4745527870_9850deb0ee_z.jpg

Bernkastel
https://upload.wikimedia.org/wikipedia/commons/thumb/a/ab/Bernkastel_BW_2.JPG/1024px-Bernkastel_BW_2.JPG

As for your train trip... It's a long one and I understand not wanting to spend a whole day en route. If you want to stay overnight, I would not go way out of my way to see more Fachwerk, but instead stay on the most direct route and just stay somewhere convenient. Augsburg, a Romantic Road town with rooms near the station, comes to mind. Ulm might work too. Both are convenient and interesting places in one way or another.

Sample Day 5... depart Boppard mid-afternoon (15:16 for example) and get to Augsburg at 20:06. That way you'd still have a good chunk of time for morning and afternoon sightseeing on the Rhine. Depart the next morning for the roughly 2-hr. trip to Mittenwald. If you were to leave Augsburg early the next morning, you'd be in Mittenwald before 10 am. Note that this is a good strategy if you are going to purchase a saver fare ticket - a single Boppard-Mittenwald saver ticket for two, with an overnight stopover, can be purchased as long as arrival in Mittenwald is prior to 10 am. So with this 15:16 departure on April 19 as an example, you'd schedule an 11-hour stopover in Augsburg and depart 7:39 for Mittenwald (arrive 9:53) If you are using a rail pass instead... to avoid using up 2 rail pass days, use only April 19, then on the morning of April 20, travel on a Bayern Ticket day pass (valid at any hour on Sat or Sun, after 9 on weekdays, €32/2)

Thank you both! Russ, it looks like we'll have plenty of opportunity to see fachwerk along the Rhine and Mosel. Looking at your suggestions, I think Ulm is more likely as a stopover for us. Thanks for the specifics on the rail travel. I wasn't aware that you can do that with Saver Fares. Is this what Lee was referencing in the post here, where the express train ticket is specific, but the regional train taken can be up until 10am the next day? https://community.ricksteves.com/travel-forum/germany/db-saver-fare

"Is this what Lee was referencing in the post here, where the express train ticket is specific, but the regional train taken can be up until 10am the next day?"

Maybe. I can't speak fully to his wording or his generalizations there.

If your saver fare ticket takes you from one German city to another German city over TWO days, then you must arrive by 10 am, and the ticket you get should be valid for TWO dates. It is not accurate to say that the train on the second day MUST be a regional train. Let's say you were traveling Boppard - Munich with an overnight stopover in Ulm. Your long-distance / express train arrives in Ulm in the evening, and the next morning you board the ICE train into Munich (7:53 - 9:13, for example.) DB sells this ticket as a valid saver fare. (7:53 probably isn't too early for some people.)

If you overnight in Ulm and you want to get to GARMISCH or MITTENWALD by 10 am - well that's a pretty long trip and you have to leave Ulm even earlier - like 6:56 for arrival at 9:54 Garmisch. Leaving at 6:12 will get you to M'wald at 9:27. That works as a saver fare too - even though the 6:56 train leaving Ulm is an IC train, and the 6:12 train is an ICE.

But no domestic saver fare with Garmisch as a final destination will be sold for arrival after 10 am, irrespective of the train type.

If you want to go to Mittenwald that 2nd day but arrive AFTER 10, you might experiment at the DB site with a ticket to INNSBRUCK. A saver fare that crosses borders does NOT have the 10-am arrival rule and travel can be spread out over about 3 days. You will need to schedule one overnight stopover in ULM and a second stopover in Mittenwald (with 00:00 stopover time) in order to get the proper routing. Then you just step off the train in Mittenwald before it leaves for Innsbruck. Try it.

Some Rhine tips...

If your nights on the Rhine include April 30, you can get in on "Wine Witch Night" in Oberwesel. At this local fest, the growing season is kicked off with the burning of a straw wine witch and the crowning of a new wine queen. Lots of ceremony, with parade, fireworks, and of course wine.

Oberwesel is only 5 train minutes south of St. Goar.

St. Goar has maybe the most scenic setting on the Rhine and serves as a good travel base town in part because of the convenient all-day ferry crossing there - it's the most convenient of the west-bank Rhine towns if you are planning to use the trains on the opposite river bank to visit Marksburg Castle in Braubach, Rüdesheim, etc. With the exception of the wine fest, St. Goar isn't very lively in April as a rule, but Boppard, which is several times larger, is easy to reach from a St. Goar base (about 10 train minutes) and offers a greater number of things to see and do if you end up with some down time in the afternooons and evenings.

Russ, thank you for the train and the town info. We like the idea of staying in a smaller town, so St. Goar will probably work well for us, and looking at info on Ulm vs. Augsburg--Ulm is the more likely stopover for us. And we just might be on the Rhine for Wine Witch Night, which would certainly be an experience!

"Is this what Lee was referencing in the post here, where the express train ticket is specific,
but the regional train taken can be up until 10am the next day?"

Maybe. I can't speak fully to his wording or his generalizations there.

On an advance purchase, Sparpreis or Savings Fare ticket, the long distance train(s) on the ticket are specific, i.e., you have to take the train specified on the ticket. However, according to what the Bahn calls Vor- und Nachlauf (before and after travel) any regional train on the ticket is not train specific. That is, you can take any train on the same route from midnight the first morning until 10 AM on the following day.

So, for instance, if you had a Savings Fare ticket leaving from St. Goar at around 9 AM by regional train to Bingen Hbf, followed by an IC from Bingen to Ulm, then another regional train from Ulm to Kempten, as I did in 2017, you could take a regional train from St. Goar earlier than 9 AM or any later regional train the got to Kempten by 10 AM the following day, but you could only take the specified IC from Bingen to Ulm.

Lee, thanks for the explanation. That makes things a lot more clear for me! Still haven't decided yet, but leaning toward a stop in Ulm on the way down.