"Finding info on earthing and touch voltages is nearly impossible". You write what I think. I would tweek it slightly to finding "accurate info". Firstly, as OC says below the resistance of the CPC in a TT system is largely immaterial. It only really comes in to play in a TNC-S
system. Reason being that in a line to earth fault in a TT system, almost the entire voltage will drop across the rod due to its relatively much higher resistance in relation to the L and the E. The "touch voltage" will thus be largely determined by the resistance of the rod rather than the CPC. However your thoughts about the same fault in a TNC-S system are valid. As the fault current flows, not through the rod, but back through the Neutral, the resistance of the Cpc now plays a much more significant role in determining the "touch voltage". Now, having "put my foot in it" by posting to late on a Friday evening I want to avoid doing it again on a Saturday. I wish you all a good weekend.