You can go from Calais to Veurne on https://www.openrunner.com/r/9794574. From Veurne, go to Bruges, then along the canal to Sluis & up to Breskens to take the ferry. The default route generated by https://www.fietsnet.be/routeplanner/default.aspx is quiet good for this bit. After the ferry, use the route from the Race around the Netherlands up to the German border https://ridewithgps.com/routes/29581779
. I never cycled the bit through Germany, but of course there is the north sea cycle route you can use for that stretch.
You can go from Calais to Veurne on https://www.openrunner.com/r/9794574. From Veurne, go to Bruges, then along the canal to Sluis & up to Breskens to take the ferry. The default route generated by https://www.fietsnet.be/routeplanner/default.aspx is quiet good for this bit. After the ferry, use the route from the Race around the Netherlands up to the German border https://ridewithgps.com/routes/29581779
. I never cycled the bit through Germany, but of course there is the north sea cycle route you can use for that stretch.