Cook shrimp on hot grill or grill pan for 2 minutes on each side until pink. Remove from the skillet and set aside.
Cook the pasta
Cook pasta according to package instructions until just al dente. Remove pasta from water by scooping out with a slotted spoon into colander.Return pasta water to stove and blanch broccoli for 5 minutes. Strain broccoli into a colander and rinse immediately with cold water to stop broccoli from cooking.
Make the alfredo sauce
Melt butter in a medium saucepan over medium heat. Add garlic and cook for 1 minute, stirring frequently, until broccoli is lightly browned.
Whisk flour into the butter garlic mixture and cook for 2 minutes. Gradually stir in milk, whisking constantly to prevent lumps. Cook the alfredo sauce for 6 minutes or until mixture thickens, stirring constantly.
Stir 1 cup freshly grated Parmesan cheese, cream cheese, and salt, stirring with a whisk until cheeses melt.
Toss sauce with hot pasta and broccoli.Top with cooked shrimp and sprinkle with remaining ¼ cup Parmesan cheese, a squeeze of lemon juice, and lemon zest.Season with salt, black pepper, and red pepper flakes (if using).