You decided not to tell us at what time you arrive in / depart from Calais. This is important to know as it will dictate how far you might go before arriving at your hotel for your first night. In turn this then starts to dictate your second night and eventually what time you might arrive in Prague.
Me? I’d maybe look at something like:
Calais to Luxembourg or somewhere like Trier (fist night) - anywhere between Diekirch and Trier, really. Diekirch is about 275 miles / Trier is about 320.
Next night in say, Bayreuth or Nuremberg or, if you cut north more, say Jena. Diekirch to Bayreuth is 317 miles. Trier to Bayreuth is about 45 miles less…. I guess you can see where this is heading.
Bayreuth to Prague is 180 miles.
How long it will take you to ride these distances on “Most scenic route/roads would be preferable” ie avoiding motorways, is up to you. Not least it will be dictated by when you start each day (see the comment about departing from Calais), what time you finish each day and how often you stop for wild flower pressing. That is sort of what Hustler is saying, too.
Now it’s your turn:
1. You know you much better than we know you.
2. As Hustler suggests, put Calais to Prague into Google maps and look at the simplest (fastest) time and distance A to B and break it up into the time / distances YOU can do in a day, dictated by the time you depart from Calais and your mid-afternoon arrival in Prague. That gives you the basics.
3. Having broken it up, put the first day into something like ViaMichelin or Kurviger. These are quite good as you can dictate if you leave out motorways / toll roads or leave them in. The Pro version of Kurviger allows you to change routing for individual segments within one overall route, which can be handy.
4. Does ViaMichelin / Kurviger throw up some possible ideas? How about if you sacrifice some part of “Most scenic route/roads would be preferable” for some (not all) motorway sections during each day?
5. Can you travel over to France the afternoon before, rather than arriving in Calais I guess, early doors (but it might be mid-morning or even lunchtime). If you can, great. It will save you an hour (time difference) and the lost crossing time on day one. That saved time can make a big difference to day one (where your hotel might be) and, affect day two and three.
The truth is, with such a long distance, there are several ways to skin a cat, as Neil W’s and now my posts show.