If you still consider riding from Berlin southwards it would make sense to contact a branch of the ADFC (German bicycle touring club). They offer maps specifically for bike tourers, showing the most adequate & interesting routes for cycling. Their homepage:
www.adfc.de Unfortunately only in German
A very interesting possibility would be to check out the cycle trails along the main European rivers. They are signed out like most of the overland trails. These routes usually go along small roads with little or no traffic.