Does anyone know how you can modify the bitrate output by the rtmp encoder. The standard output is very high,  which gives a good quality stream but it can cause problems server side in our app - streemin.  Thanks

Simple answer, you can't.

The live stream is simply processed by the resolution, frame rate and data rate of the picture transmission. The resolution, frame rate, and data rate are decided by the transmission.
The resolution, frame rate, and data rate of the Android and iOS app live broadcasts are basically the same as the all-in-one machines and vary with the parameters of different aircraft.
For example, the P4 resolution and frame rate are 720P@30fps, and live stream is almost 720P@30fps.
When the Mavic image transmission is selected for the normal mode, the data rate of live broadcast is basically the same as the quality parameters of the displayed image transmission.  RTMP protocol has the same resolution, frame rate, and data rate. You cannot manually select the live data rate unless you change the image transmission data rate of the aircraft. Hope that this information would provide clarification in regards to your query.