After rendering is finished, I can use onAfterRender to hide the custom loading indicator. But which event can I use to show the loading indicator before the rendering starts?
Usually, you'll want to display the loading indicator when you initiate an HTTP call that loads the data. This way it will cover the whole waiting time.
I'm already doing this. But I want to show the loading indicator for the time between assigning the resources and events to the scheduler and for it to rendering them
You can do that at the moment when you assign the data. There is no special event for that as this is always a manual action.