Finally, after numerous iterations, I think I have figured out my 2020 tour.
I will follow the Eastern German borders.
First, I will cycle from Heidelberg eastbound to Görlitz at the border to Poland and the Check Republic. Then I will turn North, following the German-Polish border until reaching the Baltic Sea. Then, I will follow the coastline westbound until I meet the former border between the two Germanies. From there on, I will go South again. 2.500km and the areas should be quit rural and lovely.