No problem.
I simply put Calais to Athens into the reasonably good Kurviger app and asked it for the three alternatives, all whilst sat in the pub. It’ll be a great jaunt, whichever way you go and is certainly doable within the 14 days outwards journey timeframe you’ve allowed.
https://kurviger.de/en
The daily estimated mileages of 250 (no motorway) and 350 (with motorways) are probably not unrealistic on a day-after-day basis, if that’s your thing. You’d definitely get time off somewhere along the line to either not ride the bike at all or to stop off to do ‘stuff’, all well within the fortnight’s timeframe you have set aside.
Do let us know what you do decide on and how it goes. Very few here do big milages, the Alps and the Pyrenees being the general stopping point. This though is understandable, as few have the luxury of weeks away from home and work.
PS Don’t follow the Kurviger route(s) exactly. The algorithms are good but not perfect. They sometimes take silly little detours, than no sane person would use. But, as a guide and for suggesting the direction a route might take (the Pro-version in particular *) it is amongst the best out there.
* Good in that you can, all within one dingle route, divide parts as direct (motorway) and parts non-motorway, plus varying the relative ‘twisty’ qualities of the non-motorway sections.