I recently bought the DFRobot isolator modules and tested them against the capacitor hack. The issue I have found with the capacitor hack is that while they are successful at removing the galvanic offset, the choice of capacitor affects the EC reading. The use of the capacitors injects additional capacitance into the measurement circuit which needs to be charged/discharged by the a.c. sense current. I found that if I use the 1uF capacitors as described in my hack, the readings tend to be limited around 2000 uS/cm, and if I use 15pF capacitors the readings tend to be stuck around 10-20 uS/cm. Evidently the capacitors are limiting the EC signal.
I removed the filtering capacitors and repaired the connections, then plugged in the isolator module between the EC board and the ADC. Now I am able see readings above the limited values up to the full EC range.
Unfortunate that the capacitor hack doesn’t provide full EC range. I recommend that you try the isolator module as well.