Sorry I can't help - I'm responding because I spent some time today looking for a "dumb" running gps that just tells me how far I've run without trying to connect to anything else. It seems they've stopped making them. I think theres a real need for some simple open source hardware that can support fitness applications without being part of some saas gouging or data collection "product".

https://gadgetbridge.org/ might do the trick, but I've never used it so no idea how "serious" it is.

I use gadgetbridge with my pinetime and it's solid. The fitness tracking is fairly basic, just heart rate monitoring, but I have hopes for more software being released along the sleep tracking side of things.

I remember that Beurer (https://www.beurer.com/web/de/produkte/active/sport-und-akti...) was once recommended because of offering a cloud-free tracker. It worked quite fine, but I eventually stopped using it. That was about five years ago, bear in mind.

I can only find a single tracker in their US store (https://www.shop-beurer.com/collections/active/products/beur...), but this is (an updated version of) the one I had.

Most Garmin smart watches can track sleep and fitness. Their categorization of sleep phases is somewhat questionable, but generally no worse than the competition. After initial setup you can prevent it from uploading to the cloud by disabling Wi-Fi and removing the Bluetooth connection to your smartphone. Activities are recorded in standard ANT+ FIT files and you can download them using a USB cable for analysis in applications like GoldenCheetah.

There are occasionally projects on hackaday where people replace the firmware of cheap fitness trackers, like https://github.com/curtpw/nRF5x-device-reverse-engineering -- you could start from there.

Apple Watch and the apple health app are privacy friendly, although not very hackable.

I wouldn’t be surprised if there’s an app to export health data in a user friendly way, but I am not familiar with one. The data sitting in apple health is enough for me personally.