@bobf I ran into a problem too. The Feather M0 native USB serial doesn't work well or at all with deep sleep modes, and seems unusable for debugging.
USB serial is not needed for the project implementation, but it is extremely convenient for debugging, so the Adafruit Feather M0 seems to be a poor choice for this sort of project. Unless one opts for a more expensive debugger such as J-Link, with the associated learning curve. So I'm reconsidering where to go with my project.
As I'm sure you have considered, a solar panel would certainly help with battery life.