How to fix oscillations in cable guard drive?
Force with opamp the guard have the same voltage as the signal wire:

This surely must be tuned for certain open loop freq response of the opamp, signal source resistance and capacitances. Adjust R2. I used a generic opamp which has GBW=10MHz.
This stands well some leakage resistance inserted in parallel with the capacitors. No idea what happens when there's a real cable.
ADD: The questioner said he tried my circuit in time domain simulation and found it oscillates. He's right. Good looking small signal frequency response proves nothing of the stability of feedback circuits, because it cannot reveal poles in the right half plane nor the effect of saturation or other non-linearities.
The problem is interesting because if a solution exists the high-Z signal source may be usable without a signal transmitter. I made another test circuit:

This time it's stable. The signal is rectangular 200us long 1V DC pulse. The circuit is tuned for 3 megaohm source resistance, the given &quot;inevitable capacitances&quot; and the generic opamp which has GBW=10MHz.
I'm afraid the circuit is too capacitive load for the source. But it's remarkably less than the capacitors alone would be.
ADD2 self-adaptive compensation of leak and capacitive loading which does not depend on the internal resistance of the signal source sounds ambitious. Seems like nobody else is interested than me. Harmful. But the next one is not a guard driver. It's a compensator and it needs no tuning for different source impedances:

It has an opamp circuit which is known as negative impedance converter. Known load C1 is compensated by inserting in parallel with it negative capacitance - the negative of C3. The opamp has GBW=10Mhz and trying too tight compensation with it makes the circuit unstable. It stands C3=298pF. C4 is inserted to push the high frequency roll-off of the opamp a little further. Without it the tendency to oscillate can be seen with smaller C3.
The signal is 200us long rectangular 1V pulse. The simulation skips the initial and shows the steady state result.
It compensates also known leakage resistance. It's R4 in the next image:

If the capacitance and leakage resistance are unknown they must be measured somehow. Making it automatically can be tricky. I skip it.