Not sure that I can help. however, I could interpret that you have had WSPR running on just the uBITX Nano with the CEC software. I was not aware of that. Would you please point me to the location of any documentation that there might be? Last I had looked on Dr. Lee's site he had given up on putting it in the base Nano, and was looking at other options for a linked processor to do the decoding/encoding.
I have made a number of contacts using my v5 uBITX with a PC running WSJT-X. For that control, I used the Yeasu FT-857 (I have been told the FT-817 works, however, I had issues with it). The TX for that setup is controlled with the CAT link using 38400, 8 data, 2 Stop, None for Handshake, CAT in USB mode with none split.
The same setup worked with a Raspberry Pi 4.