Stir ½ cup granulated sugar, ½ cup lime juice, and ½ cup water together in a medium saucepan. Cook over medium heat, stirring regularly, until the sugar is dissolved, 3 to 4 minutes. Pour into a shallow bowl or loaf pan and let cool completely.

Beat 16 ounces mascarpone cheese, 1 (14-ounce) can sweetened condensed milk, ⅔ cup lime juice, and ½ teaspoon kosher salt together with the whisk attachment in a stand mixer on low speed until just combined. Increase the speed to medium and beat until smooth and creamy, 30 to 45 seconds. Do not overbeat. With the mixer on medium speed, slowly pour in 1 ½ cups cold heavy cream and beat until soft peaks form, 2 to 3 minutes.

Working with 1 ladyfinger at a time, dip and submerge in the key lime syrup, then place in a 9x13-inch baking dish. Repeat, nestling them tightly together and breaking ladyfingers as needed to fit, until the bottom of the baking dish is covered in a single layer, about 20 ladyfingers.

Transfer half of the mascarpone filling (about 3 cups) over the ladyfingers and spread into an even layer. Repeat with a second layer of soaked ladyfingers and drizzle any remaining syrup over the ladyfingers. Spread the remaining mascarpone filling evenly over the top.

Cover the baking dish loosely and refrigerate for at least 12 hours or up to overnight. Garnish with finely grated lime zest before serving.