If you really want to see the Liebana cross you will have to take some time out of your planned camino, or go at the very end of your trip. It is in a remote mountain monastery well north of the
Camino Frances, the Sanctuary of Santo Toribio de Liebana.
You can walk there from Boadilla del Camino on a waymarked path northward, the Camino Lebaniego Castellano. That takes about 10 days, the scenery is breathtaking. You can download a guide to the trail at
www.palenciaturismo. es.
Otherwise, you can take a combination of rail and bus to the Camino del Norte town of San Vicente de Barquera, and walk along the Camino from there down into the mountains to Potes and Santo Toribio. It takes four or five days, it is waymarked and there are places to stay, you can find discussions of it here on the forum.
YOu can also just rent a car in Santiago and drive yourself there. I am not Catholic, and I have seen and touched the cruz... they don't ask you what your religion is! Be there for Mass, (it's good to be there when there's a bus group) and sometimes the priest will get out the cross and let people marvel at it individually. Let him know you've come from afar to see it, they are often very considerate.
When you've finished you can drive yourself down to Madrid and head home from there... or drop off the car in Santander and take the train or bus.