You can do the whole trip off the motorway but assuming you're crossing at Calais I would stay on the motorway until you get to the turn off the A4 to Eupen between Liege and Aachen. From Eupen you head via Blankenheim to Nurburg.
The A roads up to the Liege area are nothing to write home about and will take you a lot more time with little benefit in terms of quality of riding.
ViaMichelin will route you via Brussels to Liege but IMO you are better off going Dunkerque, Lille, Mons, Liege because Brussels can get very congested and you would lose time. The mileage difference between the two routes is negligible.