@Raoul-Jong
Raoul, sorry such a question is like looking in a glass ball. You cross about a third of West Europe. The quickest route from Utrecht to Porto is more than 2000 Kilometer. The climate condition vary.
For sample: No info from you like how many days will you calculate, 4 days, (which is quite tough), a week, two weeks only for one direction.
There are so many possibilities. Mainly along the Canal, the Atlantik coast, from NL trough B, and Normandy, Bretagne, Biscaya.
Or also a beautiful alternativ cross country of B, the Ardennes, west central of F, Loire, Orleans. (which i made few weeks ago to Bordeaux from Cologne with my Cabrio and back then via Massive Central)
In Spain also along the northern coast to the west end or partly cross country via Bilbao, Burgos, Salamaca.
There are so many options and perhaps from every proposal you will receive the author may say, this was the best tour of my life.