[The French Way] to start ... mid May 2020 ...
The earlier the better. Or from September, or even October, when it will be cooler.
To my mind,
@jungleboy hits the nail fair and square about NOT booking ahead.
And if you are fit and done your preparation right you will get over the hillock from Saint-Jean in one easy bound, and a little beyond Roncesvalles to boot.
As to preparation of both body and mind, I suggest you consider working up to:
1) 15 km before stopping for breakfast; and
2) 400 metres of elevation gain before stopping ...
A trick to consider when going up steep hills (like from Saint-Jean, and elsewhere) is to:
a) dress light - you will warm up quite soon, and the day will as well
b) take really small steps - toes of one foot land about 100 - 150 mm (4 - 6 inches) in front
c) breathe in on one footfall and out on the other (or vice versa)
d) don't stop, except to admire the view towards the top: take photos with micro pauses (practice needed)
So,
@Anto1110 , I wish you kia kaha (take care, be strong, get going) and hope to hear of your safe arrival in Compostela.