We are wondering about 1) renting a car in Barcelona and traveling through Provence and Northern Italy with a drop-off in Venice.
2) the driving conditions on the coastline of Spain and France as we go to Provence (Avignon) from Barcelona.
You may want to check on the car rental drop off charges before u go further in your planning. They can be quite costly. You can train it easily from BCN to say Marseille , then drive to Provence, etc french riveria and drop off in Nice or Menton. Then train it to Italy.
There are substantial international drop off fees when you do this. Call customer service at Auto Europe to get the amount. You may consider train or flight between countries and renting a car as you need it, dropping off in the same country. For purposes of car rental, the EU is not one country.