Earlier quoted context omitted.
There's also the option of open source fitness tracker apps that don't upload your data anywhere, i.e. Gadgetbridge which I use.
I'd never heard of Gadgetbridge, but "local storage" would certainly be my requirement. It's the same issue as smart-home devices (without voice control): from a user viewpoint there's absolutely no benefit to having them phone home.
[x] Check :)
Gadgetbridge intentionally does not have network permissions. There is a manual and periodical data export allowing you to push your data for further storage, processing, visualizing etc.