I actually didn't think of a switch. Checkbox came to my mind really fast, so I just simply went to dev tools and made the simplest concept. I agree that it might be not consistent with Flarum's style and it looks ugly, but it's proof of a concept. However, I admit that I am rather up for the preview mode button to have this functionality.
Perhaps there could be a settings at admin panel to toggle between the behavior: "Replace preview mode functionality to switch between rich text editor and markdown/BBCode?" or something like that, but shorter.