Hey, there!
I agree with dancingphalanges on making sure your related texts cover a broader scope of paradigms which allow for you to balance your essay's discussion of ways of thinking. This will allow for contrast in your ideas so choose related texts that have great links to your prescribed texts, but also differences that give you enough room to discuss different ways of thinking and how they are explored. Your synthesis will be more sophisticated this way
In saying that, I do think the range of texts you have are equally significant in how your discussion will play out. I have poems and a play for mine so I chose a film as one to demonstrate to the markers that I can also analyse visual texts. I would advise for you to have at least one visual text so the picture book or documentary would work well. A short story could also work nicely but I would definitely encourage a visual text in your related texts choices. Furthermore, as both of your prescribed texts are written, you can distinguish them by using form specific techniques for the poetry (enjambment, rhyme scheme, verse and so forth) to show a broader understanding of literary forms and features. Markers would love to see analyses for form specific techniques because they are often overlooked.
I hope this helps and good luck with choosing! Let us know what you end up deciding on
Angelina