I think it's a good idea to look at existing cycle routes. Some areas have few roads suitable for cycling and taking a route that has already been deemed to meet the requirements makes for better travelling. The German bikeline books are very good and cover an extensive network, branching through central Europe and Scandinavia from Germany. Most routes are quiet, although you will meet other cyclists along the Donauradweg and on routes to Santiago de Compostela.

Looking at your demands, I'd say the R1, from Calais to St. Petersburg, might be what you're looking for. I think it would take abt. three months to complete?