I'm not sure if this really fulfills your "without proprietary software" criteria, but Polar recently announced that they've opened up their API for developers.

I'm not sure if this really fulfills your "without proprietary software" criteria, but Polar recently announced that they've opened up their API for developers. I believe you do need to have a Polar Flow account though, so the API access works along the same lines as the Google API, i.e. using OAuth 2.0 authentication that connects to your Polar Flow account, which the watch syncs to via phone/wifi. So there's no LTE, but the watch does sync to the Polar Flow account in the cloud via phone/wifi. Their M600 watch is an Android Wear device though, so it might be possible to write an app for that to access the data directly, but I'm really not sure about that part. https://www.polar.com/en/developers https://www.polar.com/en/developers