Hello Hikoi! Excellent plan. I just did this this year, started on Easter Monday from Sevilla. Definitely book well in advance. Right now is not too soon. I booked 9 months in advance, and got a tiny room in the Santa Cruz neighbourhood, not far from the cathedral. But affordably, which was the key. They did charge the entire amount, no refunds available, to my credit card, a week before I arrived. Seems to be standard policy, as it's so busy then.
There was a problem with albergues filling up, especially when a couple of them were closed for renovations, and no notice on the stage before. (Embalse de Alcantara). I learned to phone ahead. (Got a Mobilstar SIM card, very affordably, service almost everywhere, including very isolated areas.) The camino Sanabrese through Ourense was not nearly so busy, later on, as reported by friends who took that route.
More questions? Fire away! It was tough, and lonely in parts, but well worth doing.
I have tons of photos from my walk on Instragram.com/felixBC, if you have time to scroll back in my timeline.