Looking at the last leg of your route, the two days from Locmariaquer to Rotterdam, it rather depends on whether you plan on breaking this into two equal length legs or into two unequal portions.
I’d maybe be tempted to devote the second day * to a simple (further but quick) motorway run from somewhere further away from Rotterdam and spend longer to the west. The whole lot gets pretty congested once you start getting onto the axis: Arras, Lille, Gent, Rotterdam, unless you cut closer to the coast: Abbeville, Calais, Brugge. There is not much to recommend riding through Boulogne, I always take the motorway to miss the place out, no matter which way I am going. At some point you will have to cross the river Seine, at I guess roughly Rouen. I’d advise not going through Rouen itself (if you come anywhere near the place, hit the motorway, which sort of peters out into a drag past goods yards and industrial estates) but maybe head towards the Honfleur crossing or cut across at say, Caudebec-en-Caux or Tancarville. When we go to Le Mans from Calais, down the D roads, we most often cross the Seine at Tancarville. For such a large river, there are not so many ways of crossing it. The only other alternative is to go to the east-south-east of Rouen in the gap between that city and Paris.
* What time do you need to arrive at the terminal?