Given the amount of time you’re allowing yourself I’d go through the Ardennes and Luxembourg then keep going in a WSW kind of direction until I got to the Bayerischerwald then head north along the German/CZ Republic border where there are some lovely roads and plenty of small campsites or hotels to choose from.
I always suggest this but it’s worth getting to Saxony a bit early then visiting the bike museum at Augustusburg Castle and Colditz Castle for the obvious war museum while you’re in the area. If you’ve got plenty of time allocated for the return I’d also recommend taking the short drive from the Sachsenring to Dresden and spending an afternoon and evening there.
Be prepared for some serious alcohol abuse at the Sachsenring and if you’re camping take earplugs and expect to be falling asleep during the day