Hi, I saw your post last night and it prompted me to wander down memory lane...... so thanks for that!
I rode to Santiago via the Camino Frances, from Santiago south and west to the coast (based on the recommendation of a bar-tender in Santiago), then along the coast past Santander, then Bilbao and on back to Holland.
After Santiago I had no planning done, I just followed my nose.....and the sea as much as possible.
It was absolutely glorious! Fantastic country, fantastic food, great roads for cycling, amazing campsites.
But there are hills! Lots of them!
As said above, weather is significant! There can be heavy, heavy rain and hot, hot sunshine.
If high speed, high daily distance is your thing, then you can do that because a lot of the time you'll be on good, quiet roads.
But that part of the world lends itself to slow, absorbing riding.
You'll get ideas of possible routes from checking out Camino resources ( there is a route Porto to Santiago and Bilbao to Santiago)* Good for ideas and also possible accommodation options.
* For anyone thinking of cycling a Camino, it is my belief that it is generally understood to be a contemplative (yet social!) experience and depending on your route, you will be travelling along paths mainly populated by walking pilgrims. While a bike has as much right to be there as anyone else, I believe there's a special place in Hell reserved for those cyclists who fly down the paths with no regard for the walking pilgrims. There are invariably roads close to the Camino paths if you want to go fast. I say this as someone who cycled, walked, carried, and dragged a fully loaded touring bike along the Camino Frances.
In any case, good luck avoiding the hills!
It'll be a great ride!