Bottom line first: Guardian Monitor on the Apple Watch Series 3 is nice, but drains the battery too fast.
My second attempt was using the third party Guardian Monitor app, which has Apple Watch support. It's pretty sleek!
But the communication has to go through Medtronic's CareLink servers, which means the network looks like this:
There are now six nodes and a web server, just to transmit a few bytes 30 centimeters to my wrist from the sensor in my arm. That's pretty stupid. It creates a bunch of opportunities for failure, and it drains the battery of the iPhone in roughly half a day. The watch also dies in a day or two.
So I'm working on something else. Stay tuned!