It's a good few years since I have ridden in that direction, but my preference would be to stay further south. This would make more sense if going from Calais, of course. The scenery is definitely better, albeit lumpier. So (very roughly) I would go through Belgium, cross the Ardennen into Germany, cross the Rhine somewhere around Worms, then via Nurnberg to the Bohmer Wald/cesky les. From there it's a fairly direct ride across southern Bohemia towards Brno.