Thank you for a fantastic writeup and review, the delegation, and for going the extra mile to try it out on an old phone.
That advise from @tbnfl4sun is spot on! Yes we did our best to make the app compatible even with older phones. Still we will have soon an iOS version for you guys :)

For the step count difference, the discrepancy can be between sensor readings, hardware difference, sensibility levels or algorithms. We are working on providing some more options to users so that they would be able to somewhat adjust the sensitivity levels of the HW sensors, or switch between options - whichever are supported on the device itself.

It's also interesting to learn about those 2 projects you mentioned, haven't heard about them before, but they seem exciting. I'll check them out further.

Thanks again, and happy to have you on board this project with us!