The king in this space is Pandoc. It supports most things you might need in "serious" documents: figures, tables, equations, references, citations, etc. You can

The king in this space is Pandoc. It supports most things you might need in "serious" documents: figures, tables, equations, references, citations, etc. You can even customize the TeX export template if you need more control over the output. And if you need to later, you can easily re-export the same document to TeX, DOCX, or HTML. Lately, I've also been using Quarto a bit. The default export is HTML but it also supports PDF. It's nice if you're into "literate programming", where you write code blocks (e.g. Python snippets) in a MarkDown file, and those code blocks are automatically used to generate figures, tables, etc. in your MarkDown file on export. If you want something that "just works" and are not averse to commercial apps, I can recommend giving "iA Writer" a shot. It has a built-in preview and export solution that is lightning fast and has good support for equations. But I wouldn't use its export for anything important, as the output is clearly draft quality, and the MarkDown support lacks features like citations. (Though it supports footnotes, so for informal documents you can abuse that for citations.)