Bring 2 cups water to a boil in a medium saucepan over high. Stir in rice and 1/2 teaspoon salt, and return to a boil. Reduce heat to low; cover and cook, undisturbed, until rice is tender and water is absorbed, about 20 minutes. Remove from heat. Let steam, covered, for 15 minutes.

Meanwhile, whisk together mayonnaise, sriracha, sesame oil, 1 tablespoon vinegar, and remaining 1/4 teaspoon salt in a large bowl. Transfer 1/2 cup mayonnaise mixture to a small bowl; set aside. Add kani to remaining mayonnaise mixture in large bowl, and toss to combine. Stir in masago (if using); set aside. Stir together cucumber and 1/2 tablespoon rice vinegar in a medium bowl; set aside.

Fluff rice using a fork. Drizzle remaining 1/4 cup vinegar over rice; using a wooden spoon, gently fold to incorporate vinegar. Divide rice mixture among 4 bowls; top with evenly divided kani mixture, cucumber, and avocado. Garnish with furikake and scallions; serve with roasted seaweed snack sheets and reserved mayonnaise mixture.