Beru, welcome to the forum.
That's an impressive first post. For just a few months of lurking, you seem to have a grasp of concepts that many beginning bakers take a long time to learn, such as the weighing of the flour and water, baking directly on a stone and the importance of a quick bake time.
I'm also impressed by the amount of oven spring you were able to achieve with a whole-wheat-ish flour. Whole wheat tends to be the enemy of volume.
My primary advice to you is to walk before you run. Both whole wheat flour and starters are advanced pizzamaking. First, master traditional, yeast leavened, white flour NY style pizza and then graduate to specialty flour and sourdough.
When you speak of a 'Milanaise' flour being local, it is safe to assume that you live near Milan? Do you have access to white bread flour? If memory serves me correctly, Europeans generally don't have access to American flours, but, with a little investigation, they can get their hands on Canadian bread flours. That's what I'd look for.
Gluten is developed during refrigeration. With overnight refrigeration, you don't need anything close to 250 folds at the start. 250 folds will most likely produce window paning. If you reach that point on a dough that's going to be cold fermented, you've gone too far. The dough balls should be lumpy and have a cottage cheese appearance before going into the fridge.
As far as trimming the bake time goes... a 5/8" cordierite stone is good for baking bread at lower temps, but at 550, 7 minutes is about the best you're ever going to do. You either have to trick your oven into exceeding 550 (frozen towel, insulating the thermostat, cleaning cycle, etc.) or purchase a more conductive stone. 1/2" steel plate is what I recommend. The nice thing about steel plate is that it's available just about anywhere in the world.
