Hi, Brendan,
I'm not a GPS expert by any means, but I do carry downloaded tracks when I walk remote caminos. I have never downloaded the tracks for the Primitivo because it is so well marked, though. I have found that the best website for tracks for camino routes in Spain is wikiloc.com. That is the site that most Spaniards seem to use. If you go there you can find tracks for the Primitivo. You can download and then transfer to a GPS.
The problem with downloading one track for the entire Camino Primitivo is that the device is limited in terms of the number of "waypoints" it can incorporate per track. So the longer the track you download, the fewer points per km (and thus less accuracy and detail) you are going to get.
That's why you're going to find that the tracks on the website are all in shorter stages. There's a good search function though.
I will add some unsolicited advice here, sorry. I have seen many people walking with their heads in their GPS. I didn't want to be that person, so I made "rules" for myself. Keep the device in my side pocket on the pack. Remove it only when I am at an unmarked intersection or when I have been walking for a long time without seeing an arrow. And never use it on well-marked caminos. I remember last year that when I finished up with the Ebro/Castellano-Aragones/San Olav and got to the Salvador/Primitivo, it was a bit unnerving at first not to have the GPS, but there is really no need for it and walking without it is much more fun, IMO.
Buen camino, Laurie