Have you tried the "Fake It" setting under the Split section of the Radio tab? Here is the explanation from the WSJT-X manual:
"Split Operation: Significant advantages result from using Split mode (separate VFOs for Rx and Tx) if your radio supports it. If it does not, WSJT-X can emulate such behavior. Either method will result in a cleaner transmitted signal, by keeping the Tx audio always in the range 1500 to 2000 Hz so that audio harmonics cannot pass through the Tx sideband filter. Select Rig to use the radio’s Split mode, or Fake It to have WSJT-X adjust the VFO frequency as needed, when T/R switching occurs. Choose None if you do not wish to use split operation."
You might want to check your BFO setting is the drop off is occurring at 1,000Hz. Should be at 500Hz.
Here is the video from Ashhar Farhan on v6 calibration.
I would first just check the current BFO setting with the BFO Tuning Aide. Only do the last BFO set, not the first or frequency calibration steps, if you see the "plateau" is significantly out of the center of the two bars. In your case, If the drop-off is caused by the BFO calibration, it will be to the right of the center.