Grilled Fish Tacos

Name: Grilled Fish Tacos
Ingredients: 2 garlic cloves, finely chopped; 3 Tbsp. fresh orange juice; 1 tsp. dried oregano (preferably Mexican); 2 cups chopped white onion, divided; ¾ cup chopped cilantro, divided; ¼ cup extra-virgin olive oil, plus more for grill; 5 Tbsp. fresh lime juice, divided; 1 lb. tilapia, striped bass, or sturgeon fillets; Kosher salt, freshly ground pepper; 1 cup mayonnaise; 1 Tbsp. milk; 4 corn tortillas; 2 avocados, peeled, pitted, sliced; ½ small head of cabbage, cored, thinly sliced; Salsa Verde; Lime wedges (for serving)