The rating of a sleeping bag that you need will be influenced by you being a cold or a warm sleeper. If you are usually cold at night, as I am, I chose a bag good to +3 or +5 C. If you are warm at night you can be comfortable in a bag good to +5 or +7 C. But remember that there are lots of options...
Most albergues have extra blankets so if you think you will be cold, grab a blanket. Or, you can sleep with your socks on, or in your extra clothes.
I have walked the Via de la Plata and the
Camino Frances in April / May and found that the temperature in the albergues varies with the elevation as well as the weather. Roncesvalles, O'Cebreiro and San Juan de Ortega are at quite high elevation and were the coolest. Other nights, it was so warm that I slept with my sleeping bag half unzipped.
Enjoy your year of preparation,
And try not to fuss about the details.
everything will work out fine.
David, Victoria, Canada