Avocado Veggie Sandwich

Name: Avocado Veggie Sandwich
Ingredients: 1/3 cup rice vinegar; 2 tablespoons water; 1 tablespoon granulated sugar; 1 teaspoon kosher salt; 1/2 cup red onion (thinly sliced ); 1 Calavo avocado (pitted and peeled); 1/2 cup canned chickpeas (rinsed and drained); kosher salt; freshly ground black pepper; 1 pinch red pepper flakes; 1 teaspoon fresh lemon juice; 4 slices bread; extra virgin olive oil (for drizzling); hummus (for serving); sliced cucumber (for serving); shredded carrots (for serving); marinated quartered artichoke hearts (chopped, for serving (see notes)); quick pickled onions; alfalfa sprouts (for serving); feta cheese (for serving)