I absolutely love Adafruit tech for low-volume hardware hacking projects. Particularly the Feather nRF52832 [0]: onboard flash, Bluetooth Low Energy, a 5V-to-3.3V converter, and USB battery charging all in one little off-the-shelf board. And the runtime [1] and bootloader [2] are totally open! I'm tinkering with my own little fork to add signed updates, lock down the OTA DFU mechanism, and switch to a more secure BLE pairing mode.
By the way, if anyone knows of an alternative to the RedBear Nano V2 [3] in terms of form factor and power profile, please let me know, since Particle is sadly killing that board off after buying out RedBear. Glad I don't have to worry about that with Adafruit.
[0] https://www.adafruit.com/product/3406
[1] https://github.com/adafruit/Adafruit_nRF52_Arduino