Any suggestions for itinerary from Frankfurt en route to The Netherlands would be much appreciated.
How much time do you have for the trip? Driving or train?
In Germany, Cologne and smaller Rhine towns are obvious candidates along the most direct route to Amsterdam. Or it's not difficult to detour via Brussels, Bruges, and more of the Netherlands, if you have a few days. How to Look Up Train Schedules Online gives you the DB train schedule link and tips for using it. This will show the fastest train connections between any two towns, unless you enter specific "stopover" points.
5-7 days going by train.
Thanks for your suggestions. They are working well and falling into place nicely.