Heat the oven to 350°F and coat a 13x9-inch baking pan with non-stick spray or butter. Set the prepared pan aside while the batter comes together.

Pour the syrup from the peaches into a bowl, then blot the fruit dry on a cutting board before chopping it into small pieces. Sprinkle the flour over the peaches and toss until the pieces are lightly coated.

Combine the cake mix, oil, eggs, and reserved peach syrup in a large bowl using a hand mixer or whisk. Stop mixing as soon as the batter looks smooth to avoid developing a firm crumb.

Fold the floured peaches through the batter with a spatula. Use gentle strokes so the fruit stays intact and remains spread through the mixture.

Transfer the batter to the greased pan and bake for 25 to 30 minutes. A toothpick inserted in the center should come out with a few moist crumbs rather than wet batter.

Allow the cake to cool completely, then pierce the surface all over with a fork. The cooled cake should feel set enough to hold the holes without tearing.

Stir the peach preserves and 1 tablespoon water together in a microwave-safe bowl, then heat for 10 seconds and stir. Heat for another 10 seconds if needed, stopping when the glaze looks glossy, fluid, and smooth.

Pour the warm preserves mixture evenly over the cooled cake and brush it across the surface. Work it over the fork holes so the peach glaze can soak into the crumb instead of pooling in one area.

Beat the cream cheese at medium speed until smooth, then add the vanilla and powdered sugar while starting at low speed before increasing to medium. Scrape the bowl as needed so the mixture becomes lump-free and evenly blended.

Pour in the heavy whipping cream gradually while mixing at medium speed, then raise the speed slowly once the mixture begins to thicken after about 30 seconds to 1 minute. Stop as soon as stiff peaks form and the frosting keeps its shape, since continued whipping can make it grainy.

Spread the finished frosting over the completely cooled, glazed cake and add the desired garnish. The topping should look soft and pillowy, with enough body to hold swirls from the spatula.