The payasam will thicken as it gets cold, you can just warm it in the microwave to loosen it a little. You can add more coconut milk and heat it real quickly if you want it thinner. Keep in mind that adding coconut milk will reduce the sweetness.
You can adjust the amount of jaggery and coconut milk, to your taste.
I personally like to use jaggery that is darker in color, just so I get that nice dark brown color for my payasam.