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 ...
