I'm with
@SYates in thinking to do it at the end.
But proposing in front of the Cathedral right when you first arrive might not be the best time. It takes away from what is already an emotionally loaded moment, and something that deserves to be savored on it's own. But why not afterwards, once you're basking in the glow of having finished the camino journey - and looking ahead at the journey of life.
You could go
back to to the Plaza do Obradoiro and pop the question there some time later.
And...not to over-think it. A proposal is a very private moment and not theatre. So why not play it by ear? There may be a natural moment that arises which would be far more perfect than something planned ahead.
A beautiful sunset, or even the opposite end of the spectrum - like when things are so awful that it's funny and you loose it in giggles about being wet and muddy and plugging up some horrible hill. In spite of the physical misery, these can actually be very happy times and times when heart bonds are fired in the kiln of struggle together.