The service conductors entering the CT cabinet are parallel 500 each phase from PCO. Then one single set of 500 to 400 amp panel and a single set of 4/0 to 200 amp panel and only one 4/0 grounded neutral to 200 amp panel also the 4/0 has a tap for equipment grounding to 400 amp panel
I'l try one more time
Where is the system will your GEC originate ???
If your POCO supply is (2) 500s then you have to size a common GEC based on 1000 kcmil which will be a 2/0 Cu connected to the grounded conductor termination in the CT cabinet (based on 250.66 )
The detail may change slightly if you are connecting at a point other than the CT cabinet.
IMO, regardless of the point of connection, the GEC to the water and steel needs to be 2/0 min based on 1000 kcmil of service conductiors (#6 mion to rod, $4 min to UFER).
If you decide to originate your GEC at your disconnects you can tap to the 2/0 with a 1./0 from the 400 amp and #2 from the 200 amp panel based on the 500 and 4/0 feeding them (see the sketch in the referenced thread)