Even simpler Markdown-based: https://www.getoutline.com/
I'm currently rebuilding the editor; My goal is it have an easy WYSIWYG editor that allows instance back-and-forth switching to Markdown. One of the tricker parts is avoiding obscure/custom markdown syntax for non-common/custom content blocks, as one of my main principals is to ensure user content is portable/non-proprietary.
BookStack does support Markdown content editing although it is WYSIWYG or Markdown, jumping between the two isn't really supported (Yet, Hoping to achieve this later this year).