Privacy respecting fitness watch (device)?

I am looking for a device which could provide some useful training feedback (the more sensors the better), that is respecting my privacy and does not need an account or my data to be sent to someone else. Everything should be on my device. Does not need to be a watch, any device is welcome. I have not issues tinkering with hardware/software, so something custom is welcome too. Any suggestions?

Ps. I have tried the galaxy watch4 but as soon …

device fitness privacy watch