Poach peaches: In a medium saucepan over medium-high heat, bring 1 cup water, the granulated sugar, saffron, cardamom, and rose water to a boil; boil 1 minute. Set peaches flat in syrup, reduce heat to medium, and cook 3 minutes. Gently turn fruit over. Cook until barely tender when pierced, 2 minutes more. Transfer to a plate, let cool, and chill about 30 minutes.
Meanwhile, reduce poaching liquid over medium-high heat to a scant 1 cup, 2 to 5 minutes. Let cool; chill about 30 minutes.
Make mascarpone: In bowl of a stand mixer, whip mascarpone at medium speed until light and creamy, 30 seconds. Add whipping cream, powdered sugar, vanilla, and cardamom and whip until thick and fluffy (but don’t overwhip), scraping bowl as needed, about 1 1/2 minutes. Fit a large pastry bag with a 1/2-in. star tip and fill with mascarpone mixture, or just use a spoon.
Assemble trifles: Trim crusts from cakes and slice lengthwise 1/3 in. thick. Cut 12 rounds using a biscuit cutter about 2 3/4 in. wide; save extra cake for other uses.
Gently push a cake round to bottom of each of 6 glasses about 3 in. wide and 3 1/2 in. tall. Spoon 1 tbsp. poaching syrup (including spices) over cake. Next, pipe a layer of whipped mascarpone over cake (or spoon a generous 1/4 cup mixture into each glass). Sprinkle each with 2 1/2 tbsp. chopped peaches, then a generous 1 tbsp. pistachios. Repeat layers of cake, syrup, and mascarpone. Set a peach half on top. Pipe or spoon a dollop of mascarpone on peach. Sprinkle each trifle with 1 1/2 tsp. pistachios and drizzle with poaching syrup.
*Dip in boiling water 20 seconds, then peel.
MAKE AHEAD: Up to 1 day, chilled airtight.