Add new main load center for EV charging
If you supply from the 500mcm you could use the 10' tap rule as listed in the NEC 240.21.1 chargers are a continuous load so you would need to size the wire at 125% of the nameplate for the 2 chargers as a minimum (the wire size can not be below 1/10 ampacity but you are fine here) so your plan sounds good.