I did a 5000km round trip in 2015 on a supermoto: UK, Belgium, Netherlands, Germany, Austria, Italy, Monaco, France, Andorra, Spain, France, UK. Took me about 2,5 weeks including staying in Spain for 1 week.
Ask me anything...
I reckon you take the ferry to Calais or Dunkirk? My usual route was through Belgium to Germany and then country roads from Cologone all the way to Munich.
This is my general tip for long distance: don't do motorways unless you have to. Switching positions on the bike makes you not notice the mileage. On country roads you are forced to do that.
Edit: Just seen you plan to ride through France for the most of it. You have to pay a lot of toll for the motorways, it's not worth it. I recommend ride through Belgium and Germany. It's not much longer anyways and the roads are much nicer. You are skipping the Black Forest completely as well! Southwest corner of Germany. Lovely area.