I’d suggest looking at the Atlas Scientific sensors…they have one for measuring electrical conductivity across a range of salinities: https://www.atlas-scientific.com/conductivity.html. We’ve been using them with fairly good success for the last several years. They do foul rather quickly if there is iron in your system, and we’ve found that they do require periodic (if not monthly) calibration. On the up side, however, they are relatively small and inexpensive and come with available C++/Arduino code and an circuit that does all the data processing. The tip is 12 mm in diameter but the other end connected to the cable is actually a little wider so I’m not certain it will fit if you need to extend the probe some length down a well. I’m not in the office today but if you like I can measure the top width tomorrow when I return.
Good luck! I’ll be interested to find out what others suggest.