Cut the tofu into small cubes. Put the vinegar, sugar, salt, and 1 cup water in a large bowl. Whisk to combine, then add the scallions, garlic, and tofu; toss gently to coat with the marinade. Refrigerate for as little as 15 minutes or up to 2 days.
Drain the tofu mixture, reserving the pickling liquid. Put the tofu mixture in a large bowl and add the radishes, cucumber, and avocado.
Toss the ceviche with 2 tablespoons of the reserved liquid, and the olive oil and pepper. Taste and adjust the seasoning, adding more pickling liquid if you like. Sprinkle with the cilantro and serve.