Munich, Bavarian Alps, Salzburg May 2023

Some ideas all by train : Regensburg - Munich (2 nights especially the English Garden and the Victualenmarkt) - Salzburg (three nights including the castle tour and a hike along the crest of the mountains and a side trip to Berchtesgaden) - Innsbruck/Hall (2 nights with local travel to Zillertal on the Zillertalbahn local railway) - Mittenwald/Garmisch-Partenkirchen (three nights including a trip up the Zugspitze and or hiking up the Partnachklamm gorge) and back to Munich to fly out.

Have fun!

If you stay in Füssen you can take a direct bus to Oberammergau (1h 45m). To see the Zugspitze and Berchtesgaden you’ll want a car.

No need for a car to see the Zugspitze. From Garmisch-Partenkirchen a cogwheel trail will take you to the restaurants just below the summit, and you can ascend the rest on a trail and steps if you wish.

https://zugspitze.de/en/Our-mountain-worlds/Highlights/Zugspitze-Round-Trip

Or if you wish to see the Zugspitze from another viewpoint, ride the Alpspitze cablecar to Osterfelderkopf and walk out on the AlpspiX.

https://zugspitze.de/en/Our-mountain-worlds/Highlights/Garmisch-Classic-Ticket

https://zugspitze.de/en/Our-mountain-worlds/Highlights/AlpspiX

Others have provided excellent advice already.

I will add that I suggest you stay in Garmisch instead of Fussen. The Fraunhofer is a great guesthouse for lodging. Also, take the cog train up to the Zugspitze.

While in Salzburg, visit Berchtesgaden and go up to the Eagle's Nest.

Yes to Garmisch instead of Fussen. You did not mention the Castles, which I think is the only reason to go to Fussen. Not reason enough fro me.

Munich: Since museums and WW II are not important, what kinds of city-sights are must-see for you?

There's actually PLENTY to do in this area besides Munich. You might just leave a day for the "city-life" there at the end, prior to your flight out.

Rick Steves and his readers generally seem to look at the city of Regensburg as a transfer point from cruise boat to other places when in fact it is a fabulous old-world place where the streets themselves are an outdoor museum. I'd encourage at least a full day there.

https://www.regensburg.de/welterbe/en/world-heritage-site
https://www.tripadvisor.com/Attractions-g187312-Activities-oa0-Regensburg_Upper_Palatinate_Bavaria.html

Garmisch-Partenkirchen is a near-ideal travel base for seeing this region of the alps. Public transport to G-P and from G-P to other places like those below is excellent. (You could break up the 3-hour train trip between Regensburg and G-P with a stopover in Munich, where you change trains anyway. Put your bags in a Munich station locker and see what interests you.)

Mittenwald
Innsbruck, AT
The Zugspitze
Oberammergau
Ettal Monastery
Linderhof Palace
Neuschwanstein (Bus 9606 for direct trips to N'stein and Füssen, or train + bus connections via Austrian towns like Reutte or Pflach)

Here's a rail/bus map showing routes around G-P. Route 9606 to Füssen/Neuschwanstein isn't fully shown. See 2nd map (green line) for that.

https://www.dbregio-shop.de/uploads/files/RTWerdenfels_Streckenkarte.pdf

https://www.ammergauer-alpen.de/fileadmin/Mediendatenbank/Ammergauer_Alpen/Dateien/Fahrplaene/Streckenplan_der_Regionalbusse_in_den_Ammergauer_Alpen_.jpg

The Fraundorfer Inn in G-P is a great place for traditional Bavarian food and entertainment. They have rooms too. And don't miss an outing to nearby Mittenwald., only 20 minutes from G-P by direct train.

This direct link to the German Railways site will be useful for finding travel schedules from any place to any place, whether by train, bus + train, or bus:

https://reiseauskunft.bahn.de/bin/query.exe/en

I'm comfortable driving in Europe, so I'd do something a little different than most of your suggestions. I'd rent a car, making sure I had the car and paperwork for both Germany and Austria. I'd go from Regensburg to Passau along the Danube river. Then I'd stay in the valley but head upstream along the Inn and the Salzach into Salzburg. Then see Berchtesgaden and Chiemsee, and then to Garmish-Partenkirchen, see the Zugspitz and Castles, and return the car in Munich.

It would cost more. But there's a reason I'm not listing all the familiar tourist stops along the way. If you do this you will get sidetracked by the small towns and scenery. You will more than likely spend a night or two in some unplanned gasthaus in a place you've never heard of. You'll eat in places people on this board never have, so there won't be any recommendations. You'll have time to get out and wander, to sit and watch the river, to shop local, and in fact, to actually experience something unique.

I'm Alps and nature focused. Take a look at Samerberg and the surrounding area (Hochries). Traunstein is nice and Reit im Winkl are too. Farther west Fischen im Allgäu and Oberstdorf. You can dip down into Austria. With a car it's not that far. I'm an hour north of Regensberg and can get to Innsbruck in four hours. Take a look at some of the valleys (Tal) in Austria. Sorry. I don't use guidebooks. I agree with just let it happen. You have the time.
https://www.foto-webcam.eu/webcam/hochries/

Thanks to al for the timely, quality replies. As I mentioned we will be using Rick's book, but we want to keep an open mind and consider all options. Thanks for sharing your experiences ! Don't stop, I'm still in the process of putting this trip together !

Steve