That's as far as I've gotten with my itenary, Amy ideas?
Rick Steves' itinerary suggestions for Germany are prioritized for the "average tourist" by the number of travel days one has. His 21-day rampage through Germany contains some good destinations, but to include all these places would be an exercise in perpetual motion. I see you have Berlin on your wish list; IMHO you should ignore Rick's fantasy that one can leave Salzburg (or wherever) for Berlin on the morning of Day 8 and finish with Berlin by the end of Day 10. If you choose to go to Berlin (and you don't have to) you should plan on at least 4 full days there.
At least Rick tells you to "slow down" at the very end:
3 days: Munich, Bavarian Alps
5 days, add: Rhine Valley, Rothenburg
7 days, add: More of Bavarian Alps, side-trip to Salzburg
10 days, add: Berlin
14 days, add: Nürnberg, Mosel Valley
15 days, add: Dresden
17 days, add: Baden-Baden, Black Forest, Trier
21 days, add: Würzburg, and slow down
1.) You are newcomers to European travel - and there's a learning curve to European sightseeing and getting around. For a less stressful time, don't pack your weeks and days too tightly with travel deadlines.
2.) It's also unnecessary to "cover" the whole country geographically from north to south. You could easily spend 3 weeks in just 1-2 of Germany's 16 "Länder" (states) and have a great time. And less ground travel of course helps one stay within a budget.
3.) Don't "adopt" an itinerary or even specific destinations. Go places that look right for you. Like every traveling couple, you and Elizabeth are not "average" - you surely have your own interests and intellectual curiosities, your own activity levels, your own preferences for leisure time... Where are you on the spectrums? Do you live on great food, or on history? Are you museum lovers or haters? Big city people, or nature lovers? Your answers to questions like these should partially guide your destination choices.
4.) For an enjoyable and educational time in Germany, it is really not necessary to focus on the big tourist "hot spots" or the places recommended most strongly by the guidebooks - or to squeeze in as many of those as possible. Lesser known places can be just as enjoyable (or more so.) Looking at Rick's list, I would nominate Nürnberg (and neighbor-city Bamberg,) the Mosel Valley / Trier, and Würzburg (plus surroundings) for your consideration. Each of these would rank higher than Munich on my list of personal preferences.
5.) The Rhine and Bacharach: The Rhine is where you will find a multitude of castles and castle ruins from medieval times (the Füssen area "castles" are actually palaces that are far more modern.) MARKSBURG Castle in Braubach on the Rhine is the real thing - never destroyed - and a must-see if you are really interested in castles. BURG ELTZ (Mosel Valley region) is also outstanding. St. Goar's RHEINFELS Castle is a worthwhile set of ruins with a good museum. BACHARACH is a small, very sedate town with the prettiest old-world buildings on the Rhine. But as a base town, it comes with drawbacks... one of which is train noise (note location of the tracks in photo.) For easier river cruising, closer proximity to castle tours and the Mosel Valley, riverfront lodging/dining, convenient ferry crossings, quieter hotel nights, and more things to see and do, consider basing in attractive Boppard. Nearby St. Goar is a good alternative as well.