How can I vary the output voltage of a "smart" USB charger?
There is no "smart charger" that would allow you to negotiate output voltage to a fraction of volts, to 5.3V as you need. At most you can have 5V, 9v, 15V, and 20V. There could be an optional mode for custom voltages, but this is likely beyond normal reach and requires implementation of full Power Delivery protocol. You don't want to do this. Even in the simplest case of QUALCOMM QuickCharger you would need to provide certain sequence of voltages on D+/D- to get it into 9V mode.

Power delivery across Raspberry Pi 3B is kind of under-engineered, so better cables cannot possibly solve the voltage droop problem across the Pi.

Your best option is to take a standard 5V adapter, open it up, and adjust the feedback circuit for 5.5V output - this level is the official top for USB VBUS power, and it will give you some margins for your overly-sensitive WiFi dongle.