Hi Nina,
I have walked on the section between Santillana Del Mar and Baamonde, which is where the del Norte approaches the Frances route. It IS a beautiful camino and I also walked in October (2021). Keeping in mind that Covid 19 may have depressed the numbers, it was not overly crowded, or even crowded at all, at that time of year. I think is a very good nexus between still warmish weather, places being open and not too many people vacationing at the coast in October.
There is a bit of road walking, but many opportunities exist for taking beautiful paths along seaside routes
that are off road. So, don't necessarily be deterred- I expect you will be rewarded with enough company of fellow pilgrims, at a really nice time of year along the north coast of Spain.
I would have continued on the del Norte, had it not intersected with the Frances and the tremendous crowds in the last 100k of that route. Instead, I transferred by train to Pontevedra, on the Portuguese, to finish on that route. Good luck which ever way you go!
I hear and read that the Irun to Santillana Del Mar section is both strenuous and breath taking, but have no personal experience there.