Cut watermelon into 2–3 cm cubes. Pat lightly dry if very ripe and wet.
Halve cucumber lengthwise. Scoop out seed core with a spoon. Slice into half-moons.
Dice avocado into 2 cm pieces. Toss immediately with 1 tablespoon of lime juice to prevent browning.
Slice red onion paper thin if using. Soak in cold water 5 minutes to mellow.
Whisk lime juice, lime zest, olive oil, sweetener, chopped mint, garlic if using, salt, and chili flakes together until combined. Taste and adjust.
Add watermelon, cucumber, and drained red onion to a large bowl. Pour two-thirds of the dressing over and toss gently.
Add diced avocado. Fold in gently with a large spoon — do not break the pieces.
Scatter crumbled feta and torn mint leaves over the top.
Chill 15 minutes if time allows.
Just before serving, drizzle remaining dressing over the top. Add extra feta, mint, lime zest, seeds if using, flaky salt, and cracked pepper. Serve immediately.