I cannot answer from experience, but I will be starting out from Oviedo on the 25th, along with my childhood friend for whom this will be a first camino (my many-eth camino). I’m staying away from the section you describe on the Frances because I’ve been there at that time of year and by Sarria the crowds were overwhelming, and those were not Xacobeo years! The heat was also pretty brutal until beyond Ponferrada and the start of October.
I’ve walked in very cool weather on the Portuguese, and extremely hot, July weather on the Frances…
I’ll take cooler, even with some rain.
For a sense of the weather, use the “Accuweather” long range forecast. It’s based on pervious averages I think, but the 14-day forecast will be very accurate, and the longer projection a very good indicator.
Whatever you choose, it will be right for you.
Buen Camino.