I would recommend getting either a:

• USB-C [3.1 Gen 2|3.2 Gen 2] - 10 Gbit
• USB-C [3.2 Gen 2x2] - 20 Gbit

The first one is of the same age as all the ROG Phone 2's ports. The second one should still be backwards compatible, but may not be on store shelves.

It is also possible that a first generation USB-C cable was shipped for charging. But even if it wasn't, a first generation USB-C cable should still charge (though it is possible such a cable might charge at a reduced speed, if they in fact DO use parts of the USB 3.1 spec... which we do not know :P)

• USB-C [3.0|3.1 Gen 1|3.2 Gen 1] - 5Gbit

I see no way to determine the generation of USB-C cable that was shipped with the 30W charger when the cable is completely unmarked, apart from one of those USB-C testers which I've been really itching to pick up!