We highly suggest installing Tow-Boot to the SPI Flash.
Having the firmware installed to SPI makes the device act basically like a normal computer. No need for weird incantations to setup the platform boot firmware.
Alternatively, starting from the Tow-Boot disk image on eMMC is easier to deal with and understand than having to deal with U-Boot manually.
To configure default console I/O to use serial instead of HDMI (default):
hardware.rockpro64.console = "serial";