Mix peach soaking syrup ingredients until sugar dissolves. Set aside.
Slice layer peaches, toss with sugar, rest 15 minutes. Drain and pat dry.
Beat room temperature mascarpone until completely smooth.
Whip cold cream to soft peaks. Add powdered sugar, vanilla, lemon juice, and zest. Beat to stiff peaks.
Fold whipped cream into mascarpone in three additions. Do not overmix.
Dip each ladyfinger in soaking syrup 2 to 3 seconds per side. Arrange in a single layer in the dish.
Spread half the mascarpone cream evenly over the ladyfingers.
Arrange half the macerated peach slices over the cream in a complete layer.
Repeat with a second layer of soaked ladyfingers, remaining cream, and remaining peaches.
Finish with a smooth final layer of mascarpone cream across the entire top.
Arrange decorative peach slices across the top. Brush with warmed apricot jam.
Cover without touching the surface. Refrigerate minimum 4 hours or overnight.
Dust with cinnamon if using. Garnish with mint. Slice with a clean knife wiped between cuts. Serve cold.