When you get there, go to the Turismo and ask. They will have the most up to date info. (We hit Zumaia at about breakfast time--0900--and enjoyed tortilla de patata and cafe leche.) In 2017 when we started this route, we stayed in Zarautz (two towns back) at a pension referred by the helpful fellow at the Turismo there. In 2018 when we completed it, we stayed in Getaria (one town back) and after my morning foot taping we probably hit the trail about 0640.
Most of the Turismo folks are happy to tell you of places to stay. The only one we encountered that wanted to actually make the (same day) reservation for you was in San Sebastian. The others just gave a list and map.
I see by your self description on the left that you're familiar with a lot of this stuff, so my remarks are probably more useful to lurkers who are newer to it.
Semi off topic comment: I know it's the fashion in some circles to make reservations for lots of nights even before arriving on the scene. As General McArthur is supposed to have said, "no battle plan survives first contact with the enemy." In this case, the "enemy" would be the weather, how the body reacts to the trail, etc. etc. We personally only reserve for Santiago, where there can be a crush on any weekend, or if it turns out we're hitting a place on a "hot" date.
Trust is part of this Camino game, in our opinion. St. James looks after his visitors.
If you get there and discover that the carefully planned schedule isn't working out as expected, please, please notify the places that are not going to see you after you had reserved a spot. /semi off topic comments.
Buen camino