I thought we'd have received some feedback from the OP by this time.
As it's lunchtime and I'm no longer in the middle of a field, I thought I'd spend 10 minutes looking at the Kurviger route, Calais > Stelvio > Mugello > Black Forest > Calais, with the website set to the third setting (Curvy route but avoiding cities and highways) and then chosing the Top Plus map overlay, via the sort of layed cake box thing in the top righthand corner of the map. This is about as close to the equivalent of a Michelin style map Kurviger provides.
It looks OK; definitely easy enough to do. Not suprisisingly, the outward and return routes are near enough along the same line, the curvy route algorithm for Calais to Mugello being much the same as the algorithm for Mugello to Calais, the only diferences being the slight detour to the Stelvio (I have put it on on the way down) and to the Black Forest on the way back.
But not entirely happy (and as it's impossible to break anything and it's free) I then mucked about a bit.
I changed the Kurviger curvy route to go: Calais > Mugello > Stelvio > Black Forest > Calais. This put the detours to Stelvio and the Black Forest both onto the return route, not one on the way down and the other on the way back. Again, not surprisingly, this changed what was near enough a single line into something maybe more interesting.
Calais to Mugello is a quite nice line that now goes roughly (the towns are close to the route, so used to identify only): Amiens, Troyes, Dijon, Besancon, Geneva, Turin, Mugello. It's the way I'd go if my intention was to go from Calais, as it is direct enough to do in a sensible timeframe (the intention being to watch a Moto GP after all) and leave enough flexibilty to pick up motorways if I was short of time or the weather was really dire. That being said, there is often far more mind numbing spray on a motorway in heavy rain than there is on a decent N or D road.
The way back, Mugello > Stelvio (assuming it's open) > Black Forest (whatever that means, as it's a huge area) > Calais is OK. It goes roughly: Mugello, Modena, Verona, Davos (the home of the world's cheapest coffee *), Schaffhausen (stop off for the Rhine Falls, perhaps?) Freiburg-im-Briesgau, Metz, St Quentin, Calais.
As Kurviger will easily deal with circular routes, the OP can do exactly the same thing in Kurviger with no problem, displaying them on one map. Zoom in and out. Use a bit of imagination and then work out how to get it from there on to a GPS device. My suggestion on the latter, do not even think of doing it as one single 2,000 mile route! Break it up into sensible daily stages, one stage for each day of your time away. Go to the GPS section if you get stuck! Am I going to recommend how to break-up two thousand miles into days and find a hotel at the end of each? No.
The more I look at Kurviger, the better it works. Is it perfect? No. Do the routes it throws up need checking, particularly if it's intended to load them into a GPS device or if the rider really does not need every twist and turn A to B? Yes, very probably.
Looking at this entire thread, it shows why 'rip-off' tour guide companies charge 'rip-off' prices to guide bods on motorbikes over 2,000 miles from Calais to Mugello and back, along interesting routes. If you are going to do it properly, then it's not easy, not least as they do don't know anything about a customer's expectations, riding abilities and whether they like five star accommodation or Formula 1. It takes time and effort, two qualities that many people are apparently short of (or are keen to avoid) hence the "Me and my mates want to go down windy roads for two thousand miles.... and we leave at...." The internet and GPS devices are supposed to have made things easier. To a very large degree they have made it too easy, as all someone has to do is type, "Me and my mates....". At the same time it seems (to some at least) to have made it incredibly hard,
because they do not try. The world survived and thrived before UKGSer and GPS devices, as we all bought a map and used our imagination. Imagination is a powerful tool, give it a go; it's free at least.
Await the OP's words.....
* I lied about that bit. It's bloody expensive!