Cook the diced peaches with brown sugar and cinnamon in a skillet over medium heat for 6–7 minutes, until softened and syrupy; scrape the skillet so the peaches heat evenly and look glossy. Transfer to a sheet pan and let cool completely (no warmth left), so the base won’t melt when layered.

Stir graham cracker crumbs with melted butter until evenly moistened and sandy; set aside at room temperature so it stays crumbly.

Beat the heavy whipping cream to stiff peaks, until the cream holds sharp ridges when the beaters are lifted, then fold in the sweetened condensed milk and vanilla until smooth and thick.

Pour half the ice cream base into a loaf pan and spread into an even layer for a flat starting base.

Layer half the peach mixture over the base, keeping the syrup from pooling on one side by distributing the fruit evenly.

Sprinkle half the crumble over the peaches in an even scatter so every bite gets a buttery crunch.

Repeat with the remaining base, peaches, and crumble, swirling gently with a knife for marbled ribbons instead of fully mixing.

Freeze for at least 6 hours, or until firm, until a knife inserted near the center comes out with no wet streaks and the surface looks set.