Instead of creating yet another custom syntax, I‘d suggest to support CriticMarkup. CriticMarkup is supported already by quite a few apps, and it‘s well designed & thought thru (so that it doesn‘t clash with any other syntax, yet is still easily recognizable).
With CriticMarkup syntax,
{==this would be highlighted text==}
{>>this would be a comment which would be left out in preview<<}
Here‘s a somewhat contrived example:
