Hi, Taylor,
You gotta love these pioneers who want to trailblaze new routes! I assume that the walk from Pamplona to Zarautz would be all asphalt, even though probably on minor roads most of the way. If that's not a dealbreaker for you, that's great. But if you think your feet will not tolerate the pounding, there are some dedicated trail connections between the Norte and the Frances. They would have much much less road walking. The Baztan, as you've already seen, requires a lot of backtracking eastward. So would the Vasco Interior between Santo Domingo and Irun.
But there are two other options that go more directly north-south. Stay on the Frances a bit longer and walk to Mansilla de las Mulas. From there, you take the Vadiense/Liebana Caminos to San Vicente de la Barquera and continue on your merry way on the Norte.
Or stay a little longer on the Frances, all the way to Leon, and head north to Oviedo on the Camino del Salvador. At that point, you can decide whether to keep going north to join the Norte in Gijon, or whether to walk the Camino Primitivo from Oviedo to Santiago.
There's information on all these options on separate sections of the forum. Buen camino, Laurie