I have a very good friend who tried to walk the
Camino Frances with me in 2014. He has serious knee problems and decided rightfully so that Orisson was his limit. He's a true competitor and his older brother and I bragged about our walks on the Camino and many of it's tributaries. After hearing us share our stories of the walk and our trumped up tales of the agonies and ecstasies, etc.; he wanted to see SdC, the Botafumeiro swing and go to Finisterre.
So we drove from Biarritz and returned in one weekend in 2015. We drove the Autopistas most of the way but there were still several opportunities to see pilgrims trekking; but if you
really want to see the "pilgrim's way" from a car begin with following the N-135 from SJPdP to Pamplona , then pick up on the other National Routes that were originally the route of the Camino, ending with the N-547 into to Santiago.
You will be able to make this a road less traveled as far as automobile vacation and you could include side trips to many historic sites rarely seen by pilgrims limited by their foot travel (See The Pilgrims Road to Santiago by Gitlitz and Davidson) and visit many regional vineyards and wineries. You can stay in small villages with pilgrims and vicariously share their experiences over dinner. Five days, no problem!
Car rental has usually been tricky in Europe. You will pay a heavy surcharge for a car rented in one country and returned in another. So if you plan to return a car in Spain make sure you rented in Spain. This from an expat who hasn't rented a car in Europe since 2010. So maybe you can find a better way to do this.