@CalebJohn This plugin works like a charm, thank you for your efforts!
Since you already added highlight rendering and strikethrough, for the sake of completeness I would like to suggest to add also the ++ synthax, or insert. I know that it's niche thing, I myself use it seldom, but it would nicely complete the rendering of the formats already available in Joplin, since the ++ syntax can be activated via the "markdown" preference pane, just like ==. It could be rendered as an underline, to be consistent with the preview pane.
Thank you again!