I have done a similar our from Zeebrugge, but since that went through Rotterdam, you could do either. The key is two map books:
Bikeline Rhein-Radweg 3, ISBN 3-85000-001-X at scale 75K which gives cycle paths from Rotterdam to Mainz
Vlanderen Fietsroute, ISBN 90-5934-050-7 at scale 50K which gives cycle paths essentially right round Belgium.
These don't quite join up, but the roads and cycle paths are friendly enough, and you can make up whatever you like based on these two resources.
Bikeline, see
Verlag Esterbauer has hundreds of brilliant cycle tour map-books all over Europe - mostly in German but if you can't handle some German you are likely to have a hard time anyway - you can cycle all the way from Rotterdam to the Black Sea on their cycle path maps.
Possibly further than you wanted just now.
regards, Rod