Add 3 cups frozen strawberries, ¾ cup syrup, ¾ cup water, and 1 tbsp lemon juice (if using) to a high-speed blender. Blend until smooth, pausing to push the strawberry sorbet down if the blades get stuck. If too thick, add 1 tbsp of warm water at a time until it blends smoothly.
Transfer the strawberry sorbet to a metal loaf tin or other freezer-safe container and place in the freezer.
Repeat these steps with the remaining ingredients, then serve immediately or store covered in the freezer for up to 1 week.