I have worked with a lot of home irrigation equipment including both AC line-powered relays and a little with battery-powered 9vDC latching relays. I understand you are interested in learning MySensors, Arduino, etc. It appears that you could easily get a reliable wired commercial irrigation controller and valves working quickly and at low cost. The main reason to use this approach in my opinion is your major requirement - reliability. But also, speed and cost.
I have worked with hobby electronics for many years, and many hundreds of hours with MySensors and related electronics, and thousands of hours of software development. I use commercial irrigation controllers because I would never be able to build one as reliable even if I had unlimited time to do it. Even if building, writing code, and testing was done, something designed and built wll not have controls, display, and functions anything like a commercial unit.
I suggest you make your first MySensors/etc. project only for the sensors you want. This is challenging enough for your first project since you mention you are not experienced with electronics. It is better to learn to walk before you learn to run!
One example of this difficulty: have you bench-tested using an Arduino to activate/deactivate a latching 9vDC irrigation valve? They are not at all like an ordinary relay. If you can do this, I am impressed at what you have learned.
In your situation suggest you put a commercial AC mains to 24vAC powered irrigation controller (USA $50-$100) at your house with a cable to your valve box, and use non-latching 24vAC valves. If you must use 9vDC battery-powered valves, or must locate the controller underground (no way to use buried cable) you can buy commercial battery powered valves with built-in control for each valve (USA $25-$50 each), or a commercial battery powered controller for multiple valves. (brands: DIG, Orbit, Rain Bird, Hunter).
I apologize if it sounds like I am trying to discourage you. I am not! But I encourage you to start with a simpler project as it is hard enough to get your first working project even if you are only building one sensor or relay!