I did Distinctively visual for Standard English too
I did war poems. tbh, it doesn't really matter what supp texts you choose as long as they have a distinctively visual focus.
Your thesis in Distinctively Visual should always be about HOW the techniques in certain texts CREATE images in our mind which FORCES us to see the world as the composer sees it. e.g. in war poems, language techniques are utilised to make us visualise the soldiers dying like cattle, shells exploding, etc.. which makes us see how brutal war is => this in turn allows the composer to communicate the message across to us that war is futile and immoral and we respond by paying tribute to the soldiers, acknowledging them, etc.
I did the Shoe Horn Sonata for my core text so war poems made it easy for me to relate the two together as the composer in Shoe Horn Sonata also had the similar purpose of making us sympathise with women POWs so that they are acknowledged like the male soldiers.
tbh, I can see how Sherlock Holmes contains many language devices that are "distinctively visual" but i think it would be hard to write in your essay about how Sherlock Holmes relates to your core text and how he forces us to see the world from his perspective...