"However, as far as I know, the Pi can only be powered via USB C" "The Pi needs to be able to draw at least 3 amps" - I wonder where people get these alternate facts or why they choose to believe them rather than the official documentation.
It is certainly possible; I run most of my Pi from a single supply.
That is not to say there are not issues; first adequate power supply, adequate cabling, reliable connections, availability of suitable connectors etc. and the ability to construct the cabling, but it is possible.
No matter the power source, getting the Pi to power external HDD is challenging (the performance of the Pi USB power is underwhelming), and it best done with a powered drive or hub (and in this case finding a suitable hub is itself a challenge).
See Raspberry Pi Power Limitations