As far as I’m aware, the use of Maths in Focus is beneficial when acquainting yourself with the concepts of different topics, whether it is through consulting the explanations of particular concepts, or when attempting questions relevant to those concepts (like you are currently doing as assigned homework for instance), allowing you to become familiar with Mathematics Extension 1 content.
On the other hand, and consistent with your suggestion, Cambridge is considered to be appropriate when seeking more challenging material that extends beyond the difficulty offered by Maths in Focus. If you aim to maximise your performance in Mathematics Extension 1, using Cambridge would be a good idea. Additionally, if possible, you may wish to concurrently use both textbooks instead of just one of them, as follows:
- Using Maths in Focus as you first encounter a particular concept can assist you in familiarising yourself with that concept.
- Subsequently, using Cambridge will enable you to master that concept, thanks to the exposure to more challenging material, which constitutes good practice.
I hope this helps!