Preheat oven to 375 degrees. Combine sugar, cinnamon, and salt in a large bowl; set aside 3 tablespoons mixture. Peel and core apples; cut into quarters, then cut crosswise into 1/4-inch pieces. Toss with lemon juice; stir into sugar mixture with apricots and breadcrumbs.
Brush 1 sheet phyllo with butter, and sprinkle with 1 teaspoon reserved sugar mixture. Top with remaining 5 sheets phyllo, layering with butter and sugar mixture. Scatter filling on phyllo, leaving a 1/2-inch border. Starting with a long end, roll up to enclose filling; place strudel, seam side down, on a parchment-lined baking sheet. Brush top with remaining butter; sprinkle with remaining sugar. Bake until golden brown and cooked through, 45 to 50 minutes. Let cool on a wire rack 10 minutes. Cut into slices; serve warm with whipped cream.