For clarity, does the Rock5B contain a voltage stepper, to step that voltage it gets from the USB-C Power Delivery which is either of 20V, 18-19V, 15V, 12V, 9V or 5V?
Does the fusb302 chip you referenced do voltage stepping too?
Just posted a separate thread about watt supply to the PCIe M.2 slots: Rock5B: PCIe power watts? (Total, and on the 12V and 3.3V)