If the OP looks at Malcopopolo’s holiday, he can see some features which he might well be able to adapt.
1. The outward journey is broken up into seven individual stages, arguably too long for the OP. But:
a. It does go all the way from Calais to Dubrovnik, right at the southern tip of Croatia
b. It’s maybe longer (distance and time wise) than it needs to be, as Malcopopolo wanted to travel through some specific places. For example, he includes the iconic Stelvio pass. He also favours a non-motorway route. That’s fine, it was after all Malcopopolo’s holiday, he can do whatever he likes
2. The return journey up from Dubrovnik to Calais, is broken up into five individual stages, two less than the outward leg. It takes a different direction up through Croatia, than that taken downwards. The five day spread arguably suits the OP better.
3. The individual distances are quite long, but:
a. The OP doesn’t have to copy them exactly
b. He could, with not a lot of effort, amend the individual stages to better suit his itinerary. For example, he could miss out the Stelvio pass or drop in some motorway, replacing the non-motorway roads taken by Malcopopolo
c. He maybe doesn’t need to go all the way down to Dubrovnik. He could go to other places in Croatia; in other words he’ll need a map, in order to plan his own holiday. For example, if he stayed in the north of Croatia, he could maybe consider parts of this:
https://www.ukgser.com/forums/showt...and-a-couple-of-the-islands?highlight=Croatia
Or
https://www.ukgser.com/forums/showt...ia-etc-Alpentourer-magazine?highlight=Croatia
The key thing is that the bones of the journey Calais > Croatia > Calais, have been done for him or, indeed, anyone else who wants to make the same journey. Instant (or nearly instant) holidays from the pages of UKGSer….. all for the price of a subscription to the forum.