Ok so I have worked it out. Following your guidance it logically meant that the issue is with the robocopy command itself and not the hardware or TrueNAS. After 2 min of research I found that removing the restartable switch (/z) increased the throughput from 58Mb/s to nearly 2000Mb/s (2Gb/s). Amazing improvement.
I thought for future readers this may be important.
Thanks again for your help, much appreciated as I wouldn’t have probably worked that out myself.