If you travel on an ICE from Frankfurt to Berlin, a reservation could be required. On several other legs, Berlin-Dresden or Munich-Frankfurt, depending on the time of day, a reservation might be recommended. For Nürnberg-Rothenburg, Füssen-Garmisch (Zugspitze), and Füssen-Munich, reservations will not even be possible.
Dresden is not that far from Nürnberg. Instead of doing Berlin to Dresden as a day trip and then Berlin to Nürnberg, I'd go to Dresden, spend the night there, then go on to Nürnberg. You can get an advance purchase Sparpreis ticket from Berlin to Dresden for as little as €29 for two people on a Eurocity (EC) train. Dresden to Nürnberg is often just as fast with a Regional Express (RE) or Interregio Express (IRE) as with an ICE/IC/EC. A Sachsen-Ticket and a Bayern-Ticket, for €28 each (€56 for both of you) will cover your trip from Dresden to Nürnberg for both of you by RE/IRE.
You can find all of this information on the German Rail schedule website.
Several of those days can be done far cheaper with point-point tickets and regional passes (Länder-Tickets) than by rail pass. Rothenburg ob der Tauber is inside the Verkehrsverbund Großraum Nürnberg (Nürnberg Metro district). A Tarifstufe 10+ Tagesticket Plus (day ticket), for €14,70, will cover both of you for the round trip from Nürnberg to Rothenburg and back in a day. If you use on of the high speed REs from Nürenberg to Munich and regional train to Füssen, the entire trip can be done with a Bayern-Ticket for €28 for both of you after 9 AM workdays. Füssen to Munich can only be done by regional train; a Bayern-Ticket will cover your travel as long as you wait until after 9.
As for Munich to Frankfurt, you can get a Sparpreis fare for as low as €49 for two.