Since Ohm’s law is still existing this will result in massive voltage drops. How low can we go? 3V?
With 5V @ 6A at the power source you get either 5V at the PMIC or 6A but never both 
Asides that if all that has to be powered is ROCK 5B with RTL8125BG in 2.5GbE mode and for example a JMS585 SATA card in the M.2 slot why shouldn’t 5V @ 2.5A be fine?