OK, then I would suggest another alternative that is device based and not expensive:
How about the same control logic but now using the H801 RGB LED Controller? Basically have the control relays be driven by the H801.
The H801 gives you 5 independent variable channels to control. You just convert the variable voltage output to energize a simple relay. @erocm1231 smartapp allows for very fine control in the milliseconds over each of the channels. So you can easily configure 1800 milliseconds into the channel to basically give you the on-off relay action. I just tested this setup for you and it works.
I know it seems a little weird because the device is designed around RGBW lights but the flexibility of the device handler and smartapp by @erocm1231 gives you what you want. Maybe @erocm1231can chime in here to make sure I am not missing anything concerning the exact timing issues.