BTHRM and running dynamics

Yes, you could use Puck.js for this - but again it's probably something you'd have to write all the software for yourself. It's unlikely you'd be able to export all the accelerometer data to some magic app (especially at the Puck doesn't have enough memory to store a large period of high speed accelerometer data).

Next idea: Another bangle strapped to the ankle could do the trick also instead of a puck

I feel like that would be the ideal approach - the Bangle is also much better designed to deal with repeated water and vibration than the Puck.

It feels like if you exported the accelerometer data you should see a pretty clear repeating pattern that you should then be able to write a simple algorithm to work off of (that could run on the
Bangle).

I can't say it'd work for everyone, but if it works for your running style I imagine it'd keep working reliably.

I should just add that stride length should actually be pretty obvious to work out from existing data (it's just distance travelled / steps), and could just be added as an update to the Recorder's downloader app in the app loader.