If you want to only qualify for the the Compostela you can start in Valenca/Tui, two lovely small towns, separated by a river, the first in Portugal, the other in Spain. For that you are looking at 5 or 6 stages depending on how you chose to break off the distance.
There is also the Variante espititual which ads a daay but includes a boat ride.
I would recommend you buy the latest edition of
Brierly's Guide to the Portuguese and take it from there. In the 2016 edition he does not talk about the Variante, but every albergue seems to have flyers about it. You decide to take it or not upon leaving Pontevedra.
And if you buy the book from the forum you will get a free credencial. Pay for an extra two for your friends and you're set.