Just a small update. 

As an alternative to Nordic soft device I tried to implement cadence and speed sensor using open source BLE stack. Apache mynewt with its NimBLE stack seemed to be worth giving it a try. 

The result of my trials ended up in mynewt-core repository and can be found at:

https://github.com/apache/mynewt-core/tree/5a4ea5055331f3b72cec2516aecbd0698043fafb/apps/blecsc

I also decided to switch with development to newer nrf52 chip which has a bit more flash and RAM for debug features, higher resolution ADC, Cortex M4 core and lower energy consumption.

See some new tools and components in the picture below.