I tried and succeeded to a certain point to use an old irrigation controller RainBird ESP-RZX (6 valves) together with ESP32 running Tasmota. For the moment, I can control the triacs through HA, have set timers and rules, but I also can use the controller the original way.
I identified the gates for triacs, measured voltages and connected the gates with ESP32 GPIO pins through 200ohm resistors. This way we can trigger by ESP command or by original command.
Next step is to analyze the voltages, lower the resistors values and trigger the gates only if we want to, even there’s command from original controller.
Purple wire is connected to GND - TP8;
Grey wire is connected to MV (TP42) = main valve, can be used as a 7th valve but be aware that the controller triggers this one together with any of the others;
The remaining 6 wires are connected to V1-V6 gates, TP45, TP48, TP51, TP54, TP57 and TP60
Used some leds for testing. Also used a 10ohm resistor to connect GND, not really necessary;
There is a lot more to improve, if someone is interested in this project I will gladly add info and also will be happy to learn from others.