Here be the source code!

Next steps: after being happy with the small Xiao BLE for the initial prototype phase (will it work? Of course!), I decided to use the Adafruit nrf528xx feather because of:

• I already have a couple of LCD feathers. It will make the power testing much easier than a power hungry OLED display
• I already have a bunch of IMU feather. I knew at one point these would turn useful, so I plan to stack both the display and the IMU, connect a battery and call it a day.
• Last but not least. At maximum speed, the RPM reading doesn't seem consistent. I doubt it's the Reed switch (more likely is the treadmill board to be inaccurate due to the lack of feedback), but in order to have a reference part number I also ordered a bunch of these Reed switches:

Stay tuned.