Transformer Installation Made EasySort of

How about this situation. I have a 600A 277/480V service. The primary GEC is a 2/0 ground ring encircling the building w/4 rods spaced at 6.5'. Every other steel column of the building has a thermoweld tap from the ground ring. There is a secondary 2/0 GEC to the 2" water main.

Adjacent to the main panel is a 45 KVA transformer. It is fed from the main panel to service the 120/208V load center. In order to bond the X0 of this transformer now;

The engineer asked for a seperate #2 GEC to be terminated on one of the ground rods from the X0, not practical or possible now.

The water main is not in the electric room, however there is a 5" square column next to the transformer and steel bar joists 20' up. Which would be a better choice for #4 GEC from the SDS X0?

A) Tap (irreversable) the 2/0 bare from the counterpoise system (ground ring) inside the main panel
B) Weld to the 5" square column next to the transformer (not tapped from the ring)
C) Send the #4 up the wall in PVC terminate with crimp lug and bolt to the bar joist.
D) The water main in the room next door (20') or tap its 2/0 inside the main panel.

Tapping the 2/0 in the main panel is the least expensive method but is it the best and is the existing 2/0 GEC large enough to add the #4 to it?