I've made some progress on the companion app thanks to this nice tutorial from expo that I've linked before: https://expo.dev/blog/how-to-build-a-bluetooth-low-energy-powered-expo-app

It provides not only the finished code for the tutorial, but some started code to follow along as well.

Strictly speaking (not sure if that's the right phrase), this tutorial is very similar to what I'm trying to accomplish with this project: a way for people to start with an idea for a Bluetooth smart device that connects to a phone app.

I also added an objective for myself to receive data from the watch. In terms of an open source project someone can pick up and run with, this would be a good thing to have available as a starting point.