North Sea Coastal route will take you pretty much all the way although I don't know Rozenburg. I followed that route south a few years ago. It's generally well signposted but can be a bit iffy in places so I recommend using a map.
Dunkirk has terrible (no) road signage particularly at the port so make sure you have a good map. I suggest that if you do see a sign to a village, take it and then pick up the route from the smaller place. I went around in circles for more than an hour once.
The tram line - the longest in the World - runs from here, all along the coast to very close by to Bruges and I recommend you take that route. The scenery is magnificent and the cycle path excellent and flat. Not to be missed, despite it slightly elongating your journey.
I rode this way and back in April. I kept the sea on my left until Ostend, where I picked up the canal route to Bruges. Past here you can follow the canal to Damme and then onto Sluis, Netherlands. It's very, very easy to follow the signs (you'll need a map which at least shows the town names though) to Rotterdam. Once in Rotterdam follow the signs for the Maas tunnel. From here, I cycled to Utrecht and then onto Amsterdam via Hilversum.