The D-Ticket is good for the whole country, but only on regional trains. It was designed in essence for residents of Germany for commuting and for trips of shorter distances... not for tourists attempting to cover longer distances (like some of the ones you intend to adopt from Rick Steves suggested travel itinerary. ) Using these regional trains will normally require significant additional travel time as well as more changes of train for a given itinerary.
The travel leg Freiburg > Füssen (7-8 hours on 5-6 different regional trains) is especially messy, even when
the faster long-distance trains are used.
Also, the D-Ticket would be two D-tickets (each) - or one D-Ticket plus additional p2p tickets - since it covers only one calendar month and you are traveling over 2 months.
For most of those journeys the IC/ICE/EC trains would be the normal choice of travelers. And if you want FLEXIBILITY of scheduling, then the Eurailpass for Germany, which covers those trains and all regional trains as well, is the standard option:
https://www.eurail.com/en/eurail-passes/one-country-pass/german-rail-pass
A railpass doesn't cover most inner-city transport, so a combination of pass and D-Ticket, which could also be used for certain shorter train trips, could work for you.
However, with your flight less than 2 weeks away, you are likely to find this Rick Steves itinerary problematic for other reasons (crowded trains and destinations, desirable lodging that is also near the train stations you will be using, pacing, etc.) which is why I suggested creating your own trip and using the car (despite my intense preference for train travel.)