Simon, as you indicate, much has been written on this topic. I'll share with you what my thinking was in 2017 and would be again if I were to do another spring VdlP. I started March 1st in Seville. First, I wanted to avoid the heat, as I do not enjoy walking in anything above 30C. Secondly, every spring you will read on this forum about full albergues in late April and May on the VdlP. (look back at posts from this spring) This is not something I want to worry about. March 1st was perfect for this, with just enough pilgrims to keep one company in the evenings. I will say that beginning in Seville on March 1st puts a person a little ahead of the best flowers. There were some early bloomers, but the big spring burst will be later.
Weather is so unpredictable. March of 2017 was wonderful, with cool mornings and very little rain. March of 2018 was terribly wet, with deep water requiring detours or bus rides for many stages. You can look at my blog from both 2016 and 2017 to see pictures and stories of our time on the VdlP. Buen Camino.