Melt butter with garlic together in a large pot over medium-high heat. Whisk in flour; cook and stir until mixture makes a paste and is fragrant, about 1 minute. Slowly whisk half-and-half into flour mixture until smooth. Add vegetable soup mix, onion flakes, lemon pepper, thyme, and saffron to half-and-half mixture; cook and stir until mixture thickens, 5 to 10 minutes.
Stir salmon, halibut, shrimp, prawns, and scallops into half-and-half mixture; bring to a simmer, reduce heat to medium-low, and cook, stirring frequently, until seafood is cooked through and chowder is hot, about 30 minutes.