There are occasionally projects on hackaday where people replace the firmware of cheap fitness trackers, like https://github.com/curtpw/nRF5x-device-reverse-eng

There are occasionally projects on hackaday where people replace the firmware of cheap fitness trackers, like https://github.com/curtpw/nRF5x-device-reverse-engineering https://github.com/curtpw/nRF5x-device-reverse-engineering -- you could start from there.