Preheat a grill to medium-high; oil the grate. In a small skillet, heat the EVOO and the garlic over low heat. Cook until the garlic is just golden, 5 to 7 minutes. Remove from the heat and strain the oil over a large bowl; reserve the garlic. Stir the lemon zest and juice, crushed red pepper and 3/4 tsp. salt into the reserved oil.
Brush the zucchini and yellow squash with 1 tbsp. of the garlic-lemon oil. Grill, turning once, until tender, about 5 minutes. Transfer 3 tbsp. of the garlic-lemon oil to a resealable plastic bag; add the shrimp and toss to coat. Let marinate for about 5 minutes.
Meanwhile, finely chop the reserved cooked garlic and stir into the butter; spread onto the bread and grill cut side down until toasted, about 2 minutes.
Skewer the shrimp and grill, turning once, until cooked through, about 2 minutes. Remove the shrimp from the skewers and transfer to the bowl with the remaining garlic-lemon oil; toss to coat. Serve the shrimp with the grilled squash and bread.