You can buy of the shelf USB pd trigger boards that request 12v (some can be configured to receive other voltages as well).

Use a buck converter to step it down to 5v. I'd suggest buying these instead of building them yourself.

You can use off the shelf components: usb PD trigger board and a DC-DC buck converter

You might wanna ask for 3A just to account for conversion losses, but I see no reason why this wouldn't work in theory.

Yeah you can do that, just keep in mind 12V isn’t a standard PD voltage, you’ll need a supply that supports PD-PPS, otherwise the standard PD-voltages are 5V, 9V, 15V and 20V. Then you can step it down with a buck regulator.