Roast Tomatoes: Preheat oven to 120°C/250°F. Toss 2½ cups cherry tomatoes with 1–2 tbsp olive oil and ¼ tsp salt on a lined sheet. Roast 2½–3 hours until collapsed and jammy; cool.
Roast Chickpeas: Increase oven to 200°C/400°F. Pat chickpeas dry; toss with 1 tbsp olive oil, smoked paprika, and ¼ tsp salt. Roast 20–25 minutes, shaking once, until crisp; cool.
Cook Couscous: Boil in salted water per package until al dente (about 8–10 minutes). Drain well and place in a large bowl to cool slightly.
Make Vinaigrette: Whisk olive oil, lemon juice, zest, garlic, ½ tsp salt, sumac (if using), and several grinds of black pepper.
Assemble: Toss warm couscous with vinaigrette. Add raw halved tomatoes, roasted tomatoes, roasted chickpeas, cucumbers, basil, and chopped oregano; toss gently. Top with feta and garnish with oregano sprigs. Serve at room temperature or chilled.