PineTime is a great option. I wear mine every day. The firmware is licensed under the GPLv3 and the companion app Gadgetbridge is under the AGPLv3.

The firmware codebase (Infinitime) is pretty simple C++, I was able to make a lot of cool customizations to my watch by hacking at it. Good fun if you like doing that sort of thing. Kind of reminds me of Pebble Time a little bit back when it had its moment.

Take a look at the models that Gadgetbridge supports. Your data stays on your phone. There are no servers to connect to. You should have roughly the same experience on any Android device.

As for tracking sport sessions, look at what each model offers. I use it to sync my PineTime and Mi Smart Band 7.

I really like using the features that Mi Smart Band 7 has to offer AND knowing my data stays on my phone instead of going to servers in China.

Caveat: Recent Amazfit/Mi devices have needed you to set up the device in their proprietary app first to generate a pairing key.

Garmin devices are generally better, and some of them have features to keep data locally only. There's also an open source program that you can get to handle data from various smart devices, which might work if you keep yours or go with a different brand. I haven't tried the program and don't remember what it's called, but someone will probably chime in with it.

The Garmin devices are pretty solid as well. I got a mid range one that does all the health stuff I was looking for.

Also this is a nice starting point when exploring privacy respecting products in some category

https://foundation.mozilla.org/en/privacynotincluded/

Can't recommend the Bangle.js enough. Great piece of hardware! Fully usable and customizable software, even if you're not a software dev, as some people on the internet claim you have to be

I love the pinetime from pine64. It lacks some functionality but costs practically nothing (for a smart watch) and runs completely FOSS firmware and software (there may be a binary blob or something but nothing that would be a huge privacy concern imo.) You can also put a custom os on, including wasp-os which is written in MicroPython and you can find custom builds for various features. The new release of infinitime, the os it ships with, looks great too though.

Well,u can try to use gadgetbridge.Maybe will work but not sure ur model quite new. https://codeberg.org/Freeyourgadget/Gadgetbridge

U can sell it and buy pinetime https://www.pine64.org/pinetime/

Or buying from aliexpress watches like TTGO T-Watch which are based on ESP32 and u can customize and create own programs and like OS for them using Arduino IDE where u will controll all ur code of execution https://alii.pub/6px7zl

Privacy and "SMART" anything is an oxymoron fyi., ESPECIALLY with Android.

Cheap smart watches will do the trick