Preheat oven to 400°F (200°C) and line a pizza pan or baking sheet with parchment paper.
Pulse the cauliflower florets in a food processor until they reach a rice-like texture (about 12 cups total from 2 medium heads).
Transfer the riced cauliflower to a microwave-safe bowl and microwave 7 minutes, stir, then microwave another 7 minutes; let cool 10–15 minutes until manageable.
Place the cooled cauliflower in a clean kitchen towel or cheesecloth and wring out as much moisture as possible.
Return the drained cauliflower to a large bowl and stir in 1 tablespoon minced garlic, 1/2 teaspoon salt, 1 teaspoon Italian seasoning, a pinch of pepper, and 1 1/3 cups grated Parmesan.
Add the 2 egg whites and mix until fully combined to form the crust mixture.
Divide the mixture into 4 portions and press each into a thin round on the prepared pan, leaving a slightly raised edge for a crust.
Bake the crusts in the preheated oven until golden brown, about 30 minutes.
While the crusts bake, make the Greek yogurt pesto: combine 1/2 cup Greek yogurt, 1/2 cup packed basil, 1 teaspoon minced garlic, and 1 tablespoon olive oil in a food processor and blend until smooth.
Preheat a grill to medium-high. Toss sliced zucchini and Roma tomatoes with 1/2 tablespoon olive oil and a pinch of salt and pepper.
Grill the vegetables 2–3 minutes per side until charred; transfer to a plate and keep warm.
When crusts are baked, remove from oven and preheat broiler to high for about 3 minutes; sprinkle the reserved 4 tablespoons Parmesan over the 4 crusts (about 1 tablespoon each).
Broil the pizzas 2–3 minutes until the cheese is golden and melted.
Spread the Greek yogurt pesto over each crust, top with the grilled zucchini and tomatoes, and sprinkle with the remaining 1/2 cup grated Parmesan.
Return the pizzas to the broiler or under high heat just until the final cheese melts, about 2–3 minutes. Garnish with fresh basil and serve.