The powerbank will only output 9V/12V/15V/20V after it has successfully negotiated QuickCharge or USB-C PD with the device.
In the case of Brumme-W, it will only get 5V.
Since you are in HK, you could get something like a Blitzwolf from TaoBao, will be just as good as the Xiaomi. I personally own the BlitzWolf BW-P10:
Has wireless charging too
For monster batteries, i have the HyperJuice 27000mah battery pack, has USB-C PD 100W to charge my laptop.
Notice the current is low, remember this is idle, not doing anything. It is also not the Brume-W, so no wifi. At full power, it will use much more.