Thanks for sharing this app. Actually I was looking for similar solution 3-4 days back when i decided to use my old fit band and wasn’t able to find any foss app for it. So I had to go with the official app .What i did was signed up with an alias , had to give a bunch of permissions including location which i faked through mock location but after pairing it once I just blocked it’s acces to the internet with a firewall app like pcapdroid -firewall. So I guess it should be okay atleast to not to share data afterwards on a daily basis. The app still worked without internet.
Now that you bring this app , it looks a much better solution, though I found out that since I have the mi band , it will still require pairing with the official app first for getting the server pairing key.
I will try it out if it works and let you know 
Edit : I paired with this foss app after going through the long process of getting the secret auth key from official app. The foss app does the work decently , but ofcourse the UI is not a good as the official app.