Hi there! Reading the Watermark sensor is no easy task indeed. Took us a while to get accurate and reliable data out of these sensors.
First of all, take a look at the official documentation provided by Irrometer: https://www.irrometer.com/200ss.html
Especially the pseudo-AC excitation is very important and make sure you calibrate the readings using the soil temperature.
There's quite some off-the-shelf hardware on the market already. Look for a device that has a low power consumption so you can measure throughout the year, for example:
Let me know if you have any more questions!