It’s amazing what the internet can turn up. I found this by Googling: motorcycle routes holland dutch netherlands. Just five words. Knowing that Google can usually do it with three, I tried it with: motorcycle routes Holland; it still worked.
That led me to a site
http://www.lazymotorbike.eu/riding/holland/ where the hot links were dead, so I made a jump from what I read, to Google: anwb motorcycle route; three words again. This quickly brought me to the MyRoute app’s webpage. I’d guess in all, two minutes at the most, on an iPad. Here is what I found:
The ANWB is Holland’s version of the RAC, or Royal (Dutch) Automobile Club. Years ago they apparently collected together a bunch of routes that would show off ‘The best of Holland’, which they signposted. The signs are apparently gone but a Dutch bod made a record of them all, hosted on the ever popular MyRoute app. Are they any good? Who knows? Are they worth a look? I guess so but don’t go looking for bulb fields, they have all been harvested by now.
https://www.myrouteapp.com/en/info/collection/2/The-43-ANWB-Endangered-Routes
Scroll down to pick up the routes. To download the routes in a gps friendly format, you’ll need the MyRoute app but you can create them yourself quite easily with just a little effort.
Tapping on ‘View route’, brings up a separate description. From that page, tap on ‘Use route’. Several cover the area you are most interested in. Give it a go.
PS The same simple Google of just three words - motorcycle routes Holland - throws up loads of other sites to look at, I simply chose the easiest. Surf around. It’s free, you can’t break it and they can’t touch you for it....
PPS From your Kurviger map in post #7, Germany (and by association, Holland) is near enough flat once you cross a line on the axis: Antwerp, Essen, Bielefeld, Hanover, Rostock.