Combine sugar and ½ cup water in a small saucepan over medium heat. Stir until sugar fully dissolves and liquid is clear. Remove from heat immediately. Cool completely.
Zest lemons before juicing. Juice lemons and strain through a fine mesh sieve. Measure 1 cup.
Combine cooled syrup, lemon juice, lemon zest, remaining 1½ cups cold water, and salt in a large jug. Stir well. Taste and adjust sweetness or tartness.
Prepare all fruit. Hull and thinly slice strawberries. Leave blueberries whole. Halve or leave raspberries whole. Slice kiwi thinly and halve each round.
Press fruit flat against the inside walls of each mold — strawberry slices and kiwi half-moons against the sides for the stained-glass effect. Drop blueberries and raspberries into the gaps.
Pour lemon base slowly into each mold to 1 cm below the top. Pour around the fruit to avoid shifting.
Tap molds gently on the counter several times to release air bubbles.
Cover with plastic wrap. Push popsicle sticks through the plastic wrap into the center of each mold. Or use molds with built-in stick lids.
Freeze at least 6 hours or overnight until completely solid through the center.
To unmold, run warm water over the outside of each mold for 10–15 seconds, rotating continuously. Pull gently to release.
Serve immediately or return to the freezer on a parchment-lined tray for 15 minutes to firm up before serving.