I am "asking for a friend". I think that the software choice will drive the hardware choice, but nevertheless I am asking for both software and hardware. Is that on topic, or can I make it so?
Basically, what is being sought is a fitness tracker that does not share user data with "Big Brother & the Holding Company" (AKA, Apple, Google, et al).
The requested software should communicate directly with the tracker device, without passing through any server.
That's the major point. Choice of hardware would be from a list compatible with the recommended software (for Android and/or Windows). The more features the better, but privacy is non-negotiable.
How best to ask ?