Have your cocktail hour somewhere besides the dining area.
If the weather is nice, and I’m sure it will be, take advantage of some other additional space at your venue that is available for your cocktail hour. It keeps your guests away from their tables until you are ready for the meal. When the guests have easy access to the dining area during cocktails they tend to find their seats, sit, and stay there for cocktails and dinner. By the time you are ready for the toasts, they are really restless.