I've only ever had to measure the characteristic impedance of 10g ethernet through harnesses and connectors using a VNA.
Characteristic impedance of the ethernet harness was 100 ohms +/- 5 in my case.
To minimize losses / reflections you'll want to match the impedance of the cable with the PCB.
You'll need to do some math to figure out trace width, pcb material, copper thickness, other layers (assume 4+ layer board?) and match the pcb impedance with the cable... then you can worry about the TVS diodes

J