Make the peach syrup: In a medium saucepan, combine sliced peaches, 2 cups water, and sugar. Bring to a boil over medium-high heat, stirring to dissolve the sugar. Reduce heat and simmer for 10-15 minutes, mashing the peaches with a potato masher or spoon to release more flavor and color. Remove from heat and let steep for another 10-20 minutes. Strain through a fine mesh sieve, pressing firmly on the solids to extract as much liquid as possible. Discard (or repurpose) the pulp. You should get about 2-2.5 cups of syrup. Cool slightly.
Brew the tea: Bring 8 cups water to a boil in a large pot or kettle. Remove from heat, add the tea bags, and steep for 5-20 minutes depending on desired strength (avoid over-steeping to prevent bitterness—don't squeeze the bags). Remove the tea bags. Stir in the lemon juice if using.
Combine: In a large 1-gallon pitcher, pour the brewed tea and peach syrup. Stir well to combine. Taste and adjust sweetness (add more sugar syrup or a simple syrup if needed while warm). Let cool to room temperature, then refrigerate until chilled (or serve immediately over ice).
Serve: Fill glasses with ice, pour the peach sweet tea over top, and garnish with fresh peach slices, lemon, and mint. Stir before serving as natural separation can occur.