I wouldn't worry about the assessment until you have your citizenship resolved, you'll have plenty of time to prepare while you wait for that. There would be no point taking the test until you are in a position to accept a job.
The assessors are normally Brazilian language teachers, not native English speaking aviation professionals. Unless it's changed drastically in recent years you will start with some general questions about your life and experiences. Despite being called a warm-up it IS part of the assessment. Then you'll have an audio recording to analyse, a short text, and a picture to talk about. Technical accuracy isn't important, English accuracy is. Even as a native speaker don't expect an automatic 6, your English could well be better than your assessor so give concise answers to the questions asked and don't try to show off.
I've been away from this area for a few years, but several specialist schools used to offer a mock exam. It's a good way to learn the format of the assessment.
Good luck