There are other choices, too.
If you go to Basel by train, just beyond the German border is Weil am Rhein, near the airport, where cars are available which would have no drop charge.
If you stay near Freiburg im Breisgau, but not in the city, it is likely that the village or town will participate in the Konus card programme where local trains in the Black Forest are free to use.
Driving from west to east from Freiburg im Breisgau to Nuremberg on the A5 and A6 can be challenging.
The A5, from the Swiss border up to Frankfurt am Main, turn right at Hockenheim, has perpetual road works on it and can be interesting.
The A6, between the A5 and Nuremberg can also be a challenge because of the interminable road works.
It may be an easy drive for you but my experience on both of those roads last year would say otherwise.