The mayfly can generate a square wave which can then be filtered/rounded to a sinish wave. It would still need to be amplified with a low current op-amp
For TDS trials I personally would start with something like this, though might need more research – it has methods of calibration. The qu is just what sort of value does the results give
https://github.com/DFRobot/DFRobot_EC – I haven’t found a diagram yet
The cable, for outdoors (no radio emf) doesn’t need to be the shielded cable.
My guess Titanium sensors of similar size would work instead of the defined lab sensor.