Another potential “gotcha” is the relay circuit.
Note: This all assumes that you’re still using a 12 volt pump/solenoid system, and that you know the total peak-current draw of everything in the system, as startup current can easily be 2x, 3x, or even more than the steady-state current draw. Ask, and/or get the datasheet for the pumps/solenoids you’re using.
If it were MY system, I’d use the programmable relay to drive another, larger, relay that’s actually connected to the irrigation system. Make sure the current rating of the smaller, programmable relay is at least 2x the current drawn by the coil of the larger relay. This will prevent contact burnout after a few hundred cycles-or-so. This is also a good rule-of-thumb for selecting the rating of the larger relay. Try to over-specify by at least a factor of 2x to increase the relay’s useful life.
I’d also seriously consider a 0.1 - 0.01μF 200v+ rated capacitor and a 100 - 150v, surge suppressor across the larger relay’s contacts going to the sprinkler system to absorb inductive “kick” from the water valve pumps/solenoids and any inductive pickup from EMF that is conducted through the ground. (Like buried power lines, nearby buried telephone lines, etc.)
If the larger relay is located more than a few meters, (say 8 - 10 yards or so), from the smaller one, especially if the wiring is underground, or routed near other power-carrying wiring like lighting or electrical wires, I’d add the same two protective devices to the contacts of the smaller relay.
Sorry, I can’t help you with the programming. Don’t forget to share what you find and discover!