Welcome to FT!!

I know the Northern part of your itinerary better that the Southern, so I'll give some advise where I can.

From Amsterdam it's very easy to get off the train at Cologne, which we find a lovely city, and with the train station very central you can be at a hotel and ready to explore the city in a matter of minutes.

From Cologne you could look at taking a Rhine cruise (lovely scenery with CASTLES) to either Wiesbaden of Mainz, and then catch an onward train from there. The only wrinkle is that although the KB line sells a day ticket which allows stops between Cologne and Mainz I can't make the schedule work without stopping overnight along the way - not a bad option except it eats up another of your nights.

If you don't want to do the cruise, it's still easy enough to get back on the train to continue your journey from Cologne.

If you want to go right to Munchen, you can get direct train from Cologne that takes only 4.5 hours, making Cologne-Munchen very doable as a day trip. Or, you can take a side trip from Frankfurt and do a night in Heidelberg which is a very attractive city, and only an hour from Frankfurt by train.

Another option if you want to work in a little of France would be to rent a car in Frankfurt - I'd suggest the airport for easy access - and take a side trip to Alsace for a couple days. Amazing country and the food is to die for.

I'm just now working on Munchen myself for an upcoming trip and one thing I just learned is that Salzburg is actually the last stop within the Bayern Ticket zone, meaning you don't need to deal with a whole separate ticket (if I'm understanding correctly.)