Excellent news!
The Michelin app / Green German guidebook on which it is based, is created from nothing but first hand experience, no different to the first hand knowledge that caused Michelin to edge some of the roads green and label them scenic. Similarly, the Lonely Planet type books. They didn't make it all up, I promise.
The German websites that are linked, are loaded to the gunnels with maps of routes, hotel ideas and at a push (with a bit of Google translate) suggestions on what to see. Yes, it requires effort, not necessary if someone tells you to go to Marburg; the only bit of first hand suggestion this thread has turned up for you so far, but it's early days. You are going in September, which is a month or more away. All I am trying to do is encourage you (and hopefully others) is to think outside the box a little and play about in the World Wide Web, where you are tapping into near enough the whole world's knowledge. What would you do if you didn't know about UKGSer? What might you do to find out information on motorbiking and things to see and do NW of Frankfurt? What made you decide on the corner of Germany NW of Frankfurt in the first place, instead of going to the Ardennes? You decided on it for some reason.... and now you want things to see and do there.... See what I mean. Follow your own line of thought and see where it takes you. Who knows, you might have an interest in steam trains or water wheels, maybe you'll chance upon some as you surf?
Have fun and try really hard to break it. I have never managed.
Here's a suggestion. Marburg, nice as it is, is out to the SE of the area. Not impossible but maybe a bit of faff to get to and from each day? Look at the Michelin map picture in your post. Look at the roads that look great. Look at towns on the crossroads or T-junctions on the map, that coincide with the great looking roads. Why look at those? Well, you can then go in several directions, radiating out in a circle from your base. Crossroads towns, like Bastogne, are of strategic significance for a reason, they allow flexibility in the direction of onwards travel. The towns generally generate hotels and restaurants, because bods over 100's of years have set up camp there, if only briefly like you. The same basic rules of location apply when motorbiking, trust me.
Google up the place names. Look on Booking.com for hotels. It's really easy once you start.