@swc-phil , what version of TrueNAS are you running?
Over on the feature request I posted ( Enable Mac SMB (Samba) Server-Side Copy Support by Default or Provide a Toggle in SMB Service Advanced Options - #2 by Captain_Morgan ), @Captain_Morgan mentioned that Fast Copy for SMB hads additional work done on it.
Please test with Fangtooth… 25.04. Fast copy for SMB got some extra love.
The question is whether MacOS is doing something very different. You might also need to test with the their latest version.
Also, which version of Mac OS are you running?