I have walked between Santander and Colunga in August. It was pretty busy, I had to sleep in a sports centre one night, but that was pretty fun (there was a swimming pool and basketball court). Other times I found accommodations in albergues but sometimes it was busy enough that I was walking shorter days that I might have liked, to be sure of a bed.
Transportation is fine: there are buses all along the coast if you need to bail out. Don't forget there is an airport at Oviedo (Asturias) - it seems to be off a lot of people's radar.
I didn't see bag transport, but it may exist by now. Weather is mostly very nice - look up some climate figures for more details.
It is not very hard but is 'harder than it looks' on a map of Spain - because the trail has to go in and out and up and down for the river mouths. In the old days, pilgrims got people to row them across these on boats!
Private accommodation can be a bit expensive because you are competing with tourists. Look up Gronze for some ideas of prices.
Overall, I would say it is fine, but in the height of tourism season it can feel 'not very Camino-like' - this isn't just my opinion, but that of several people I talked to. So if you have the choice of timing, maybe go June to July, rather than July- August.