Direct bury solid copper ground wire?

rgk1

Getting ready to ground solar panel rails and equipment ground wires.  I have an existing ground rod just outside my equipment shed and a few feet from newly installed combiner box.  The equipment ground is now attached to the ground bus bar in the combiner box but the box is not yet connected to ground rod.

Should I connect the combiner box to the existing rod and bring the solid copper ground wire off the roof from the panel rails and run direct buried (approx. 12') to the existing ground rod connecting panel ground and combiner box to same grounding rod? 

Would it be better to abandon the existing ground rod and put a new one where the panel ground comes off the roof and run combiner/equipment ground wire via underground conduit to new rod?

Lastly, install a 2nd rod and have separate rods, one for panels and one for equipment, then connect them together underground, and if so, via which method - conduit or bare copper?