I have been using Gadgetbridge for several years. The developers are very active, with the nightly branch being released daily. I've collaborated with some of them to debug issues and add support for new devices.
I've used it with multiple watches and a scale. While it doesn't provide insights or interpretations of your data like proprietary apps do, it offers complete control over your data. I've been planning to set up a SQLite to InfluxDB/Grafana translation layer to obtain more detailed statistics.
It'll just cache telemetry locally then send it in when you reconnect to the network