TLDR: how to use USB-C as your project power supply : not all breakout boards are created equal.
Initially I didn't even know what to search for : 'downstream connection' didn't exactly jump out yelling "pick me!". With hindsight, as with most things, it is now pretty obvious:
You need a breakout with the resistor configuration that mimics USB 2.0 for a "downstream connection".
From Sparkfun:
www.sparkfun.com/products/15100
From Adafruit:
www.adafruit.com/product/4090
For Australians:
littlebirdelectronics.com.au/...
littlebirdelectronics.com.au/...