Wear gloves. Slice jalapeños into 1/8–1/4-inch rings; discard stems and extra seeds.
Combine sugar, vinegar, garlic powder, turmeric, celery seed, mustard seeds, salt, and flakes in a saucepan.
Stir over medium heat until sugar dissolves; bring to a gentle boil.
Add jalapeño slices. Simmer 8–10 minutes until slightly tender but still bright.
Lift peppers with a slotted spoon into clean, heatproof jars.
Boil the syrup 4–5 minutes to thicken slightly.
Pour hot syrup over peppers, leaving 1/2-inch headspace. Tap jars to release bubbles.
Cool to room temperature. Seal and refrigerate at least 24 hours before serving.