Like everything else, you need to decide what matters to you most about the trip.
I agree that on one hand, Switzerland is not to be missed from a scenic point of view. But you can do that with a flight to Geneva and a train or car journey into the mountains. On the other hand, no point in adding waypoints to your already long route which are not entirely needed.
Google maps says you can do it via Switzerland in 13 hrs or so, and avoiding (to the east) in 14 or so.
Chuck in a cheeky detour to the Ring (leave the E31 just before Koblenz, looks to be no more than 20 miles off the m-way.
From what I can see,
this route will take you into Italy within reach of some cracking passes, not least Stelvio. You can do the western stuff, and the Swiss stuff, another time
What might be interesting to plot is 3 legs and 2 stopovers:
Day 1 - Ijmuiden to Nurburgring
Day 2 - Ring to Italian Lakes/passes
Day 3 - Lakes/passes to Tuscany
...and consider something completely different for the way home, like eastern France or summat. By not overdoing the driving each day, that will help it feel still like a HOLIDAY
Whatever, a great opportunity for you both and look forward to messing about with maps to help you come up with the final itinery!
. Try making a route on google map by entering the places below.
