I'm very new to ... backpacking
From my reading you have an overwhelming weight of good advice on booking ahead, or not, as you please.
My understanding of you homeland is one of almost overwhelming flatness as far as the eye can see.
My suggestion, made in the strongest language the forum will allow is "Do not start at Saint-Jean!!!"
You have less than two weeks to become accustomed to a whole lot of new ways of doing stuff and going up more than 1,000 metres of elevation spread over less than 20 km is not a good training space. From my observation it is a good space to become quickly uncomfortable with both backpacking and camino.
For you I suggest starting at Logrono. There will still be many challenges for you from there.
As you start in the northern mid-summer, I strongly counsel you to carry almost nothing at all.
I suggest:
keep it simple
two sets of clothes - wear on, carry one
merino wool tops (warm, breathes, dries quickly)
merino wool underpants
floppy hat
do
NOT take a sleeping bag - a blanket from the hostel should suffice at night
you will pass many farmacia - get what you need then
backpack as lightweight as possible
It was Einstein who said, keep things as simple as possible, but not simpler.
Enjoy
I hope to hear of your safe arrival at Santiago de Compostela.
And if circumstances dictate otherwise, that you appreciate what you have achieved and are minded to return in the future.
Kia kaha, kia mÄia, kia manawa-nui (take care, be strong, patient and confident)