Fattoush

['4 teaspoons ground sumac, soaked in 4 teaspoons warm water for 15 minutes', '3 tablespoons (or more) fresh lemon juice', '2 tablespoons (or more) pomegranate molasses', '2 small garlic cloves, minced', '2 teaspoons (or more) white wine vinegar', '1/2 teaspoon dried mint', '3/4 cup extra-virgin olive oil', 'Kosher salt', '2 8"-diameter pita breads, halved, toasted until golden brown, broken into bite-size pieces', '1/4 cup extra-virgin olive oil', 'Kosher salt', '3 medium ripe tomatoes, chopped, or 4 cups cherry tomatoes, halved', '1 pound Persian cucumbers, or one 1-pound English hothouse cucumber, quartered lengthwise, thinly sliced crosswise', '6 scallions, thinly sliced', '2 Little Gem or baby romaine lettuces, or 1 small head romaine lettuce, trimmed, cut crosswise into 3/4" strips', '2 cups (loosely packed) flat-leaf parsley leaves', '2 cups purslane leaves or additional 3/4"-strips romaine lettuce', '1 cup fresh mint leaves', 'Ground sumac (optional)', 'Ingredient info: Sumac', 'a tart', 'citrusy spice', 'is available at specialty foods stores and Middle Eastern markets. Pomegranate molasses can be found at some supermarkets and at Middle Eastern markets. Both are sold at kalustyans.com.']