Many other similar devices use all types of OSS.

You just need to make sure it fails safe. If the OS or any software hangs or crashes just make sure the thing turns off and doesn't dump all the insulin or anything.

I would want a beep or something to let me know it had disconnected. (Also a low battery alarm as well.) On the device not on the phone. And the phone should have the same thing if the device does not respond to a heartbeat signal.

But this is the mitigations I am asking in terms of risk management. Perhaps people using the app can comment if any of this is already implemented?

I have an AndroidAPS. This is how it works. If the phone loses contact for any reason the device just falls back to delivering insulin at a fixed rate as normal.