Driving from Milan, Italy to Vienna, Austria have a 21 day time slot, could you suggest a touring route.
I would get a map of the area and decide which route to take. The highway distance is 388 miles. You can drive through the cities you want to visit. There are many sights which will interest you.
Determine the mileage and set a goal of the number of miles you want to drive each day. I always plan my driving in that manner.