> it's that simple markdown is exceptionally bad. It doesn't have a uniform extension syntax or native support for pre-render transforms.

I feel like "simple markdown" is a straw man here. If you're doing anything complex with markdown you're going to pick a flavor and learn it. Many flavors of markdown can do the things described.

Here's a Hugo figure which looks easier to understand than the "ugly rST" equivalent: https://gohugo.io/content-management/shortcodes/#embedded-sh...