We don't really support MicroPython explictly at Adafruit - our drivers are for CircuitPython. However, for people who like MicroPython, we submitted a build definition!
You can build the latest version from the github or load this MicroPython 1.9.4 build we crafted for you.
Load it by following the DFU Bootloader tutorial in this guide. Follow the instructions for when you have a .dfu file.
Upon success, reset the board without the BOOT0 jumper and you will see after a few seconds the PYBFLASH disk drive appear
The Feather uses the same chip as the PyBoard 1.1 so technically anything available on the PyBoard should work on the Feather, given the pin differences
We use Dx and Ax pin names, to match the Feather markings. You can see the pin names here
SPI flash is not used by MicroPython (it's something specific to CircuitPython)