Well, I went the end of August 2016. It was a heat wave. It was 95F in Trier. No kidding. It was hot and I wished I had packed another pr of capris. As it was mine dried overnight so I washed them out and wore them day after day, hahahaha!! I would see what the forecasts are right before you leave and decide. I generally wear a pr of jeans on the plane, pack 2 pr of long pants and 1 pr capris. I would have been better off on this trip to have done 2 pr long pants, 2 capris but you never know. (I had also been in Paris the week before where, yes, there was a heatwave! yikes!)
Switzerland was blessedly cool. Others had on fleece jackets but I was fine in a SS merino wool Tee and a cotton Lands End sweater. I suspect it was high 50s/low 60sF. BUT I'm from N. Idaho, I'm old, I'm overweight and I can tolerate cold better than some. I'm not sure where you are from, but if you get cold easily I would have either a puffy jacket that folds up to nothing, a lightweight polarfleece or a lightweight merino wool jacket or a puffy or polarfleece vest.
It did rain in Muerren (at night!!), Salzburg and Hallstatt. I ~always~ take a waterproof jacket on any Rick Steves tour or in fact any visit to Europe. Not water resistant, waterproof!
My main shoes are always athletic shoes. They were fine for the trails I did in Switzerland.
In Salzburg many went to the Mozart concert + dinner but it had a dress code - no jeans, no athletic shoes so I skipped it as that was all I had. If you plan to go to this, make sure you have pants other than jeans (travel pants would be fine) and at least leather looking athletic or walking shoes.
I do suggest you do research on what you want to do in your free time in Munich, Salzburg and Vienna. There is a free afternoon in Baden-Baden. Some did the baths, I strolled along the Lichtenthaler Allee and did some birding and people watching. Plus ate gelato, hahaha!!
I loved Trier and wished I had had another day there. I arrived 2 nights before but on my full day before the tour started it was so hot I was back in the room mid-afternoon to get out of the heat. If you have the time, you will want to get in to the Dom and the Konstantin Basilica on Saturday before the tour starts as the walking tour is Sunday morning and there are services thus not able to be toured.
I am NOT saying Trier is always hot - just something to have on your radar as your travel time draws closer.
There are some wonderful art museums in Munich. I enjoyed the Neue Pinakothek - very nice Van Gogh sunflower there!
Well, heck. Here is a link to my trip report, that will be easier!
https://community.ricksteves.com/travel-forum/tours/trip-report-8-27-16-gas-tour