Prob have a read through this thread.
Seems the RockPi is fed via a NB679 buck which is 40watt max.
Which is approx up 18watt for the RK3399 then 22 for USB and the rest.
m.2 is 7watt max I think.
It might be more as confused if there are several bucks onboard or the spec changed as the PDF mentions both.
So it might be 40watt to the RK3399 and ancillaries and there are 2 more bucks for USB.
Also that is v1.3
USB-C PD for me is a bit mweh as the ‘non standard’ voltages just get stepped down via onboard bucks.
But any PSU can be used 5-20V but a few of us have a preference for 12v 5.5mm barrel transformers with a usb-c adapter as inline switches and splitters are easily sourced, plus having a 12v rail are all a bonus over what are relatively expensive usb-c pd wall adapters of no apparent advantage.
A bog standard ATX PSU can be used where a link from PS_ON to GND or switch can be added.
Generally they are of higher quality and efficiency for $ and also have all the peripheral connectors.
Or just go cheap and cheerful.
With a barrel to usb adapter.