Regarding trip I would go:
Dunkerque, Gent, Antwerpen, then E34, Venlo, Dortmund and Kassel. There is no toll charges on this route.
I've done this route many times. In the day especially at peak times Antwerpen can get very busy but apart from that it's not a bad drive.
Also I'd go over with Norfolk line (DFDS) to Dunkirk.
Prefer this route as they used to not let coaches on and the food is normally a lot better than P&O and my ferry link (ex seafrance).
Just be careful in and out the which ever port you go to and as I rule I never stop after Belgium as there are a lot of undesirables wanting to hitch a ride
And Norfolkline (DFDS) normally let you get the ferry before or the one after with no fuss.
Most ferry companies let you go early or later it just depends how full the boats are or if they are backed up due to weather.