Prepare the peach filling: In a medium bowl, combine the diced peaches, sugar, cornstarch, and lemon juice. Gently toss to coat the peaches evenly and set aside to macerate while you make the crust and crumble.

1. Make the crust and crumble: Preheat the oven to 350°F (175°C). In a large bowl, combine the flour, oats, brown sugar, baking powder, and salt. Cut in the cold butter using a pastry cutter or your fingers until the mixture forms coarse crumbs.
2. Press the crust into the pan: Take about two-thirds of the crumble mixture and press it into the bottom of a greased 9×13-inch baking dish to form the crust.
3. Prepare the cream layer: In a medium mixing bowl, beat the softened cream cheese, powdered sugar, and vanilla extract together until smooth. In a separate bowl, whip the heavy cream until stiff peaks form. Gently fold the whipped cream into the cream cheese mixture until well combined.
4. Assemble the bars: Spread the cream layer evenly over the crust. Spoon the peach filling on top of the cream layer, spreading it out evenly.
5. Add the crumble topping: Sprinkle the remaining crumble mixture over the top of the peach filling, covering it as evenly as possible.
6. Bake: Place the pan in the preheated oven and bake for 35-40 minutes or until the crumble is golden brown and the peach filling is bubbling.
7. Cool and serve: Allow the bars to cool completely before slicing. These bars can be enjoyed warm or chilled, depending on your preference.