Are you set on using soil sensors, or would you be open to using Rachio's intelligence to monitor your soil moisture via zone settings and weather data? We do offer intelligent scheduling that only waters when our algorithms estimate your zone to be "empty", and water only enough to "fill" that zone back up. This takes a bit of attention when you are getting set up, but we have many users that are very successful after that. Let me know if you are interested, and we can dive into how it works and how to get set up.
If you do want to do soil sensors, there may be some strategic ways to set them up. Keep in mind, the gen 2 only allows for two sensors to be installed. Also, these sensors work as a "pause" on all of your zones, it cannot be applied to just a grouping of zones. If your zones are relatively uniform, then you could work under the assumption that if one is dry, they all are. This will only really work if they are all about the same conditions.
Let me know what you think about these two options, and if you have any questions!