Make the Cranberry Syrup: Simmer cranberries, granulated sugar, and water in a small saucepan over medium heat until sugar dissolves and cranberries break down, 10 to 12 minutes. Stir in 4 sprigs fresh thyme. Cool to room temperature. Strain; discard solids. Make the Cocktail: Combine gin, Cranberry Syrup, fresh lemon juice, elderflower liquor, and ice in a cocktail shaker. Shake until chilled; strain into 2 glasses. Top with ice and club soda. Garnish each drink with fresh cranberries and 1 sprig fresh thyme.