Making this work without a first-party backend would require a way to regularly change configuration data on the embedded device by pairing it with the mobile a

Making this work without a first-party backend would require a way to regularly change configuration data on the embedded device by pairing it with the mobile app. Bluetooth or local wifi access point. That's do-able, but unless you're Apple, it's hard to make that a seamless experience. I went for the mandatory first-party option because it is important to me to build a device that is super reliable. Nobody wants to debug the bluetooth connection with a calendar. Running the server is cheap enough that I can keep it running for a decade or more, even after I stop selling the last device. Still, the comments in this post have convinced me that offering this as a fallback option may be a good idea, to address the understandable concerns about obsolescence.