Barcelona to Nice, via Andorra any advice?

edscoble once said the climb to andorra was a very enjoyable one. though he came from the north it still means he probably passed the same roads.

Andorra is amazingly ugly, a real insult to its surrounding I'm sad to say.
It's tax free and they can't even have decent bike shops or discount internet retailers, that should be sufficient proof of bike unfriendlyness.
The road going north from it is very busy ,following the Ariège valley all the way up to Toulouse.
What I suggest is : choose the next pass and valley eastward.
The pass is lower, the Aude valley a magestic ride to Carcassonne, again choose B roads for the Limoux to Carcassonne ride, who is a death trap.
You don't have to ride on the Canal du Midi paths to reach the seaside,
lots of quiet roads next to it.
As for the Riviera riding, I wouldn't get anywhere that area even with a car.
Use the backcountry, the southern Alps if you want a challenge.
It's very hilly and hot, enjoy peaceful rivers and lakes, make it to the Riviera and catch the train to Nice.