Checkout GadgetBridge and their list of supported devices. It runs completely offline. There is differing level of support so make sure to read the details. Th

Checkout GadgetBridge and their list of supported devices. It runs completely offline. There is differing level of support so make sure to read the details. The mi bands are cheap option that works pretty well. I've got a Skagen (really a fossil) hybrid that works with GadgetBridge, but doesn't support sleep tracking unless you use Skagen's app. But I disabled network access for their app and it works perfectly fine. Would be nice if someone wrote a sleep analysis plugin for GadgetBridge for devices like this that don't do the analysis on the hardware. Pretty happy with the device nonetheless.