The Mega2560 is extreme overkill for the task. You should keep it for something bigger, it can support touch-screen. it can be used to program chips to put in proto-boards that trigger the relays.
Boards with socket headers are made for benchwork development and the 2560 with 52 IO pins, 4 UARTs, 8K Internal RAM with the ability to add External RAM or devices like TFT displays. Why waste that to do so little?