Wash and pat dry all 6 peaches. Dice 4 peaches into 1/2 inch (1 cm) pieces and place in a bowl with 2 tablespoons granulated sugar and 1 tablespoon lemon juice; stir gently and let sit 15 to 20 minutes until syrupy.
Slice the remaining 2 peaches into thin wedges, cover, and refrigerate for the topping.
Set a fine-mesh sieve over a measuring cup and drain the macerated peaches, keeping both fruit and liquid. Pour the drained syrup into a shallow dipping dish and add 1 1/2 cups peach nectar and 2 tablespoons peach schnapps or amaretto if using; set drained peaches aside.
Chill your mixing bowl and beaters in the freezer for 10 minutes. Pour 1 1/4 cups cold heavy whipping cream into the chilled bowl, add 3/4 cup powdered sugar and 1 teaspoon vanilla, and beat on medium-high until stiff peaks form, about 2 to 3 minutes.
Stir 16 oz softened mascarpone in a large bowl just to loosen. Fold in about a third of the whipped cream, then fold in the rest until smooth and spreadable with no streaks; do not beat with the mixer.
Dip each ladyfinger into the peach soak for about one second per side, then line the bottom of a 9 by 13 inch dish with a single tight layer of about 24 cookies. Spread half the mascarpone cream evenly over the ladyfingers and scatter half the drained diced peaches over the cream.
Dip and arrange a second tight layer of about 24 ladyfingers on top of the peaches. Spread the remaining mascarpone cream over this layer and smooth flat, then scatter the remaining diced peaches across the surface and press down very lightly with the flat of the spatula.
Cover the dish with plastic wrap or a fitted lid and refrigerate for at least 6 hours, preferably overnight, so the ladyfingers fully soften and the cream firms up.
Just before serving, arrange the reserved peach slices over the top, dust with 2 tablespoons powdered sugar through a fine-mesh sieve, and add fresh mint leaves if you like. Slice with a clean, sharp knife, wiping the blade between cuts.