Amsterdam to Rouen to Caen to Mont Saint Michel

Nice one !!

Only thing off the top of my head is that the normal route for it would be Amsterdam > Paris ; basically on the Camino from Amsterdam.

Then follow the Seine to Rouen -- this is a highly traditional pilgrimage route, with multiple GR options.

The major alternative would be to follow that Amsterdam route until Saint-Quentin, from there > Péronne, then down the Somme to Amiens, then follow the Chemin d'Amiens to Rouen.

From Rouen there's a complex network of routes, but I'd suggest make some sort of way to Honfleur where the Seine reaches the sea, then > Trouville-sur-Mer > Dives-sur-Mer > Caen.

From there the Chemin de Caen, but switch over to Vires ; from there, to Avranches, then the end of the Paris to Mont-Saint-Michel route.

Or, avoid Rouen and Caen, and Amsterdam > Paris then Paris > Mont-Saint-Michel on the GR1, which goes further South via Triel-sur-Seine and Alençon. Your prospective route sounds better though ...

I have seen some maps linking routes from Nederlands to Santiago, from Wobien Doyer, in the following webs:
www.santiagoroutes.nl
www.cheminscompostelle.eu
After you cn switch to:

You might consider all or part of the Pieterpad for the Dutch portion. Here is a thread about it: https://www.caminodesantiago.me/community/threads/information-about-pieterpad.77527/

Walked Caen to MSM - beautiful, but sporadically marked and very little pilgrim support.

more info: https://pelerinsdecompostelle.com/association-sarthoise-des-amis-de-saint-jacques-de-compostelle/