Stir kimchi, 2 tablespoons mayonnaise, 1 tablespoon rice vinegar, 1 teaspoon sesame oil, and ¼ teaspoon salt together in a medium bowl until well combined. Gently fold in the flaked tuna until just combined, leaving any large pieces of tuna intact.

In a small bowl, stir together Sriracha and the remaining 1 tablespoon mayonnaise.

In another small bowl, combine sliced cucumbers, grated ginger, 1 tablespoon rice vinegar, 2 teaspoons sesame oil, and ¼ teaspoon salt.

Divide hot rice between 4 bowls. Top each with the tuna mixture, cucumber mixture, and avocado. Drizzle with Sriracha mayonnaise. Sprinkle with sesame seeds and scallions. Serve each bowl with nori sheets.