The reason you have no modulation when you press PTT on the remote device is that it is sending the CAT command TX0;
TX0; is the command to "Transmit using the microphone audio" and you really want to use USB audio. It is much simpler, once the hurdles are overcome.
That was the reason that we made the Macro button that contained
This is the CAT command for a TS-590S/SG to "use the TX audio from the USB port"
I suspect that we shall need to modify the PTT (momentary transmit so long as the button is held down)
and the T/R button (latching transmit until the T/R button is pressed again) on the RigPi that you are using to send that TX1; CAT command.
That will be the modification that TS-590S/SG users will need in order to have seamless operation.
I don't know why this was not clear to me earlier, but alas, I am still learning too!
Let me look at the notes on modifying the buttons as I don't recall that process from memory.
73 - Mark - K8MHW
'A RigPi Enthusiast!' - I am not otherwise affiliated with MFJ or RigPi