If this is a GEC and the riser is EMT, the ends of the EMT need to be bonded to the 500 KCML See 250.68 (E), Since the dwg shows transformer I am pretty sure this is a GEC. You would be much better off using SCH 80 PVC for the conduit. Your bonding jumper for the ends of the EMT has to be 500 KCML. The size for the common grounding electrode conductor is not smaller than 3/0
250.30 (A)(6). At the end of that section is the requirements for the tap conductor connection. Not sure why 500 KCML is being used but there may be an engineering reason
You're correct, this is actually a common grounding riser under 250.30(A)(6). A grounding hub can be used on the EMT to bond it directly to the riser. Not sure why anyone would spec 500 kcmil when #3/0 is all that's required.