Preheat oven to 375°F (190°C). Toss diced peaches with brown sugar, lemon juice, vanilla, and cinnamon. Spread on a parchment-lined baking sheet and roast for 25 minutes, until soft and syrupy.
Let cool slightly, then mash gently (keep some chunks). Chill for 1 hour.
Whip chilled heavy cream to stiff peaks (3–4 min).
In another bowl, mix condensed milk, vanilla, salt, and bourbon (if using).
Fold whipped cream into condensed milk until smooth.
In a loaf pan, alternate layers of cream base and peaches. Add shortbread crumbs if desired.
Swirl gently with a butter knife. Cover and freeze at least 6 hours.
Texture Hack: Stir every 30 minutes for the first 2 hours for extra smoothness.
Let ice cream sit out for 10 minutes to soften.
Scoop and drizzle with reserved syrup. Garnish with mint or peach slices.