Drain 1 (15.25-ounce) can of the halved peaches. Pulse the peaches in a food processor until desired consistency; it can be smooth or chunky. For an extra-smooth peach purée, strain through a fine-mesh strainer to remove any bits.
Whisk 1 cup cold whole milk and 1 (3.4-ounce) package vanilla instant pudding mix together in a large bowl until smooth. Whisk in the processed peaches with a flexible spatula until evenly combined. Cover and refrigerate for 30 minutes. Meanwhile, drain the remaining 1 can of peaches and cut into small dice.
Beat 8 ounces room-temperature cream cheese and ½ cup powdered sugar together in a stand mixer with the whisk attachment on medium speed, scraping down the sides of the bowl as needed, until light and fluffy, about 2 minutes. Beat in 1 cup cold heavy cream on medium speed until stiff peaks form, 2 to 3 minutes.
Stir the pudding mixture with a flexible spatula. Add the cream cheese mixture, ½ cup of the diced peaches, and 1 cup mini marshmallows if desired, and gently fold together until combined.
Transfer to a serving bowl. Alternatively, place 8 (4-ounce) ramekins, cups, or glasses on a rimmed baking sheet and divide the mixture between the ramekins. Cover and refrigerate for at least 30 minutes or up to 2 hours. Serve garnished with the remaining diced peaches and more mini marshmallows if desired.
