Preheat an oven to 375F. Place 6 cups of peaches in a large cast-iron skillet. Add ⅓ cup (75g) sugar, 1 Tbsp cornstarch, 2 Tbsp fresh lemon juice, 1 tsp pure vanilla extract, and a pinch of Kosher salt. Use a spoon to gently stir everything until evenly coated.
In a medium bowl, combine AP flour, Kosher salt, brown sugar, ranulated sugar, pure vanilla extract, ground cinnamon and ground nutmeg. Add cubed salted butter, then use your fingers, a pastry cutter, or a fork to work the butter into the dry ingredients until large crumbs form.
Evenly scatter the crumble topping over the peach filling. Transfer to the oven and bake until the filling is bubbling and the crumb topping has crisped up.
Allow the peach crumble to cool for 30 minutes to set, then serve with vanilla ice cream and/or whipped cream.