I haven't taken the route that you have taken. I did take a car from Madrid, to Toledo, Granada, up the coast to Valencia and Barcelona. We wanted to go to Tangier but decided against it at the last moment.
The Spanish roads are really easy to navigate. The toll roads can be really expensive and the regular road usually runs along it and can be more picturesque. We drove through olive trees around Madrid and orange groves in Valencia. If you are renting a car in Barcelona I would recommend picking it up when you leave the city. It is ridiculously expensive to pay for parking. We paid around 35-45 euros a day. It was crazy. In other cities we stayed at before we reached Barcelona we stayed at hotels that had free or much cheaper parking. In Barcelona we rented an apartment and had to pay to store it.
My husband and I didn't have trouble navigating anywhere. Someone told us that we weren't allowed to use a GPS. I found out later that was a lie. We drove all over with a good map.
I am a big car geek and really wanted something we don't have in the States. A seat or a VW. I ended up with a Mini One. I am really glad I ended up with a small car . I drive a Mini Cooper here. Some of the small towns were so narrow that our mirrors were inches from walls. It was crazy, but fun.
The road trip we took was one of my favorite vacations. Seeing Spain from all different regions was fantastic and it's amazing how different people are only 100 or 200 miles away. it amazed me how mountainous Spain is. It is a beautiful country.
I've done more of the Northern coast and I do prefer it, as it feels less spoiled.
If you are going through Valencia, there is an amazing playground with Gulliver. You can climb up onto 'Gulliver' using ropes, ride slides down off of Gulliver's legs, go through tunnels, etc. It's pretty cool--a good place to stretch your legs.
Granada or Sevilla could be worth a detour from your route, especially if you are into Moorish architecture.
I would confirm in advance with the car rental company that it's okay to take the car out of the country. Sometimes there is an extra fee for this.