You can try getting ypur hands on an AsteroidOS compatible watch and flash it with that. It’s basically Linux on your wrist. There is a health app here and the required sensor logging is here, which is in early development but works well in what it does, like step counting and heartbeat graph. Past days aren’t shown further than a week but they are logged, so you could probably get it by hand in terminal after accessing the watch with SSH if you need to until it’s implemented (I think it’s on the to do list).

It’s my daily watch for months now, sold my Galaxy Watch4 in favor for it. It works well, is completely FOSS, and works without internet, bluetooth, or phone. Altrough both can be enabled and phobes can be connected. You won’t find a more privacy friendly smartwatch solution really.

There is an issues with the current nightlies tho, so downloads are taken offline for now. So you need to wait for that or see if anyone has old files from before the issue.

Devs are quite active on Matrix. If you have no patience you can ask there if anyone has the files. Any kind of support is also allowed there, no matter if it’s very noob or advanced level.