How long does your calibration last? At least in my experience with trying that kind of probe, there's a need for frequent recalibration, especially during the growing season, as fertilizers are either applied or as they're absorbed by the plants, because very often fertilizers change the soil conductivity. Even the manner in which you irrigate can change the salinity of the soil either up or down, and rain can have a different effect.
Then, too, unless your probe is stainless steel, you've got the problem of probe corrosion, especially with copper probes like yours, which can also cause loss of calibration. It's a known problem. They're cheap and easy to make, and they're simple to understand and use in the short-term, but they just aren't any good for longitudinal measurements.
Anyhow, for a possible solution that would fit nicely with your project, have a look at
https://lowpowerlab.com/forum/index.php/topic,1317.msg9060.html#msg9060 It would be great to hear your thoughts on that thread. Because of common ground interest, and you're being a serious developer, you might even wish to help move it to fruition.
Regardless, I'm sure Moteino's would be a good fit to accelerate making your project wireless.