I just started to use quarto (and can recommend it). It is based on Pandoc Markdown with some extensions. One extension is called shortcodes like this (they can also be used inline):
{{< include _content.qmd >}}
{{< pagebreak >}}
Actual shortcodes (here include and pagebreak) can be defined via a plugin system in lua.
In my opinion the syntax is ugly, but you could use inline code with attributes and convert with a Pandoc filter based on attributes extension (e.g. `include _content.qmd`{shortcode}). Anyway, I think this extension is worth to mention.