In a large bowl, combine the frozen peaches, thawed lemonade concentrate, 3 tablespoons of the cold water, simple syrup, lemon zest (if using), and salt. Stir to coat the peaches evenly.

Spread the mixture in a shallow layer on a rimmed baking sheet or in a 9×13-inch dish and freeze uncovered for 60–90 minutes, until edges are firm but the center is still slightly slushy.

Scrape the partially frozen mixture into a blender (work in two batches for smaller blenders). Add 1–2 tablespoons cold water if needed. Pulse on high in 10–15 second bursts, using a tamper or pausing to stir, until the mixture is a slightly chunky, scoopable slush (about 30–45 seconds per batch on high-speed blenders).

Taste and adjust sweetness with up to 2 tablespoons extra simple syrup if desired. Serve immediately in chilled glasses with optional garnishes. If too thick, stir in 1 tablespoon cold water at a time; if too thin, freeze 15–20 minutes and re-blend briefly.