I would like to use an EFR32 to create an application that reads from a digital sensor via UART and is able to send messages on a Thread network containing sensor measurements. I've seen a lot of getting started guides and done some example applications with App Builder, but I'm a bit confused as to where to start with something like this. Any pointers or good examples to start with? I will not be using a host processor for this application. My main confusion is this: how do I use App Builder to create a Thread sleepy end device that reads sensor measurements via UART?
Ideally you would want to start with a simple application. You could do something like build a sleepy-client and then add your components necessary for your device. I would recommend then enabling the USART so that you can establish communication between the EFR32 and your sensor device. Sending data across the network then depends on fitting it within the CoAP framework.
You should also look at the Sensor/Actuator Node. It does a similar function to what you are seeking in your case. It might be a good basis for your application.