How can I change the voice for the text-to-speech function in the Kindle app for Mac?

Hello,

I am trying to utilize the Kindle for Mac's text-to-speech function (Tools->Start Text-To-Speech). Unfortunately, the issue that I am having is that the app will only read text using Apple's "Alex" voice. I have a visual disability and I find it hard to retain information with this voice and would like to use Siri Voice 1 or 2.

Despite me going into System Preferences -> Accessibility -> Spoken Content and ensuring that the correct voice is selected, it still will not play anything but "Alex".

Incase anyone recommends... I have already tried simply highlight the text and hit command+esc. When I do this, I do get the correct voice, but I can no longer do it this way because Kindle says through highlighting the text I have reached the publishers limit for "copying" the text....

This is very frustrating for a person with a disability and I hope someone can help! Thanks!