My husband and I walked from Astorga to Santiago mid to end of March last year and were very happy with the weather! We had wall to wall sunshine, cool and damp mornings and a few spots of snow still on the ground after Rabanal and O Ceb, but that is to be expected. Other than that it was warm, but not hot.
We started each day with merino short+long sleeve tops, fleece if necessary and windproof tops (pertex) when it was windy. Both carried ponchos (
Altus) but never needed them. Also both wore merino buffs and hats to start the day and took them off as we/it warmed up. Layers worked well but if it had been much colder we would have had to wear everything all the time.
We were also practically alone on the trail, but then we stayed in private rooms (it was our honeymoon after all) and never got going until about nine. We saw more people after Sarria but it was still very quiet. Suited us at the time - we had the Donkey Killer (Pradela route) out of Villafranca del Bierzo all to our selves and it was amazing! Since it was before Easter lots of places were closed, like Pilar's El Lar, the Cowboy bar, Morgade, Eireixe etc, so a bit more planning was necessary.
Most of my Galician walks have been in September and it was definitely cooler than that. Bring an extra layer and get a lightweight windtop, is my best advice.
Hope that helps a bit, I am going on the CI in April and can't wait. Enjoy your planning and buen camino!