OP, once the four of you get to somewhere beneath the line, you can ride around in just about any direction and have a good time. If proof were needed, here’s a screen shot from my MyRoute library of routes in Belgium;
I have a broadly similar library for Luxembourg and / or for that part of the western edge of Germany.
Why so many?
A. Because the area is easy to get to from Calais / the North Sea ports.
B. Because there are a lot of good / interesting roads to ride or drive. Are they all virgin smooth? No, but hey, it’s part of the fun. If you must have virgin smooth, go to Luxembourg and / or over the border to Germany.
C. Because there are enough towns / large villages, to make finding accommodation easy. But, the popular places do get busy and booked. Why? We are not the first and only people to know about the area.
D. A combination of A, B and C.
What to do now?
Between the four of you, buy a half decent Michelin map of Belgium, another of Luxembourg and one of the area of Germany closest to the Belgium / Luxembourg border. From the map(s), chose a town that gives easy access to roads that look half decent. You’ll not go too far wrong, trust me.
One tip: Try to find a town on a crossroads or T-junction. Why? Crossroads are good, as they have roads feeding off them in various directions. Various directions, gives choice. It explains why the Germans spent days trying to take Bastogne in the Battle of the Bulge and why the Americans fought hard to retain it.