Roast Tomatoes (Advance Step): If you haven't already, roast 2.5 cups (half) of the cherry tomatoes. This can be done up to 3 days ahead and stored in the refrigerator.
Cook Couscous: Prepare the whole-wheat Israeli couscous according to the package directions until it is al dente. Drain any excess water and set the couscous aside in a large mixing bowl to cool slightly.
Prepare Vinaigrette: In a small bowl, whisk together the extra-virgin olive oil, lemon juice, lemon zest, minced garlic, sea salt, sumac (if using), and several grinds of black pepper.
Assemble the Salad: Pour the vinaigrette over the slightly warm couscous and toss to coat. Slice the remaining 2.5 cups of raw cherry tomatoes in half. Add the raw tomatoes, the reserved roasted tomatoes, the roasted chickpeas, sliced cucumbers, torn basil, and chopped fresh oregano to the bowl. Toss gently to combine. Top with the crumbled feta cheese and garnish with oregano sprigs.