Agree, it sounds like just way too much unless you really want/need to be on the go and on the road the whole time. I'd pick one area of Germany and stick with it; and in a really major city like Berlin or Munich -- not both, on this trip, unless you want little else in between -- allow several days to explore the place and its immediate surroundings, a whole week in Berlin doesn't sound like too much. You could even skip both of those big cities completely, focus on Koln, Bonn, the Rheinland and over to Heidelberg, Rothenburg, Nuremberg etc. Or, head straight down towards Munich in a leisurely way, and do Southern Bavaria; or up and over to Berlin. And if you stay just in the Western edge of Germany you are actually closer to parts of Belgium and France -- Strasbourg maybe, or Champagne -- than to much of Germany, so a driving route could take in a bit of all three with much less miles. I'd save Salzburg for another trip.

Anyhow I'd say you need to focus on what you really want out of the trip -- the great airfare only gets you there, the rest is for you to prioritize. If it is scenery, the trains in Germany are a dream; you will miss a lot on the Autobahn, including the chance to meet some Germans on the same train. If you like more cozy spots with BNB's, try some of the smaller towns -- still with excellent rail service. Pack lite and take taxis if you must, but I'd never want to drive so much in this period of time.