I2C are TX3 = SCL ; RX3 = SDA (2k2 pullups needed)
UART6 RX6 and TX6 can be used directly for GPS
UART4 TX4 (PA00) is the LED pin (for telem radio)
RX4 isn´t offered so this has to be soldered directly to PA01 of the F405
In most cases starting Betaflight on time (before PX4 flashing) and carefully read the dumped cli chapter resources gives information about the pinouts.