In an old forum post, it has been recommended to place a ground trace between the tx and rx segments if they are routed parallel to each other.
original comment from the post CSX Touchpad PCB guidelines
Placing ground near Tx/ Rx traces increases the self parasitic capacitance of the Tx / Rx electrodes. Also it helps in avoiding unwanted Cm formation in the traces. We want the Cm to be formed only in the region between Tx and Rx segments and not in the traces. Also CSX sensing method is independent on the parasitic capacitance of the Tx and Rx segments but depends only on the Cm between the two segments. We usually recommend placing a ground trace between the tx and rx segments if they are routed parallel to each other.
I am pretty sure they meant the segment traces (because otherwise Tx would couple to ground I believe), but just in case there is an aditional design guideline I could try, I want to make sure. I added green and pink ground traces to a pcb between the segments and segment traces of rx/tx. What was meant in the comment was pink or green?
Your reasoning is correct. The pink traces are the recommendation and the green traces in the picture must be removed as they interfere with proper Cm formation. The ground trace is recommended between Tx and Rx traces (pink trace in the picture) to make sure that Cm is formed only at the sensor.