I’m not sure you could find a 5.2V micro USB adapter, since that would violate the USB specification.
The problem is that there is a Schottky diode between VBUS_NRF (the micro USB connector) and the schematic net VIN, which is connected to the VUSB Feather pin.
This is necessary because on the Boron only, VUSB/VIN can be used as a power input or an output. You are using it as an output, and the diode is causing the 0.2V voltage drop.
The reason the diode is there is because VUSB/VIN can also be used as a power input. In this case, the diode is necessary to prevent voltage from flowing backwards from VUSB/VIN to the micro USB connector (VBUS_NRF) and nRF52 VBUS pin. These can only take 5V, but VUSB/VIN can be up to 17V; the diode prevents the higher voltage from going to these pins.
What I’m saying is use a 5V (non-USB) power adapter and connect to to VUSB/VIN instead.