Here is a page that I found on how to connect the Nextion display to a v6 uBITX with the KD8CEC firmware installed and the tft file for the correct display installed in the Nextion display.
CAUTION! I do not have a v6.? I have a v4 and a v5 and have added Nextion displays to both.? As far as I can tell the wiring and software should work. 73 Evan AC9TU