Okay, I figured it out! The problem I was having was not the original problem I thought but actually, it was the same problem this
person was having which you helped fix @theducks, where the orignal_epub was overwriting the changes made to the file.
Thanks @retiredbiker for posting the formatting template, I have very limited CSS knowledge so that's super helpful and will make the images fit in with the text nicely!
(Also there was another reply I posted before I found the solution which is pending approval so just ignore that if you see it before I get a chance to delete it)