500 gig symetrical bandwidth increase

How is the 200E connected to the new line - through the RJ45 WAN ports or the SFP ports?  By 500 gig symetrical line, I assume you mean a 500 Mbps symmetrical connection. 

I would suspect possible duplex/speed mismatch between the 200E and the device on the other side of that 500 gig  connection - on the 200E perform the following commands on the CLI:

diag hardware deviceinfo nic <interface name>

where interface name is the 500 gig connection - look for the duplex/speed lines and any rx/tx counter errors - if there are any counter errors, perform the same commands in say 5-10 mins and see those counter errors increase.

If you are no sure of the interface name, enter "diag hardware deviceinfo nic" without the interface name to get a list of interface names. 

The fgt can only connect to the other side based on set rated speeds (eg. autonegotiation).  you can check the possible detected duplex/speed options of an interface by typing the following on the CLI:

config system interface  edit <interface name>  set speed ?

The output would be similar to the following:

auto        Automatically adjust speed. 10full      10M full-duplex. 10half      10M half-duplex. 100full     100M full-duplex. 100half     100M half-duplex. 1000full    1000M full-duplex.

I would also check or confirm the equipment on the other side of that 500 gig connection is set up properly.