the uncertainty in how "deep" or the size your ground rod needs to be has one huge wild card.......the makeup or composition of the soil you are putting it into.How deep does a rod need to be for static discharge?
I know for lighting or high voltage a grounding rod needs to be 4’. But a ground strap for fuel truck has a tiny surface area connecting to the ground for discharge.
but for static you can get surface mats that work.
So if you have a metal rod into the ground it seems a foot or 18” would be enough for static. But I don’t know that for a fact.
gravel, rocky desert soil doesn't work so well for conductivity as opposed to more fertile dirt. I have seen issues with communications gear that wouldn't work correctly, the problem traced back to a poor earth ground, even though it had an 8' traditional copper rod driven well into the ground. we moved the rod to a new location and then used water after pounding it into the ground to add moisture to the soil and all the problems went away.
without measurement equipment to actually gauge if your ground rod is even working......everything being done for safety in this area is just "due diligence" and not anything scientific or guaranteed to work.