Sorry that I can’t answer the specific question you want, namely how to do this while avoiding the command line. But what you want is eminently achievable using Pandoc’s “reference-doc” command. Search for “reference-doc” here. Here is a link to Pandoc’s default docx reference file. Note that Pandoc will pay attention only to styles in the reference-doc. So any tweaks you want to make must be made to the reference-doc in Word using Format > Style…
Here’s how I use this. I use Scrivener to compile to markdown using “Multimarkdown” upon compile. You can use Scrivener’s section template features to assign docx styles to various sections (as I describe here), and then you tell Pandoc which docx to use as its reference file when it converts the markdown file to docx. And voila. It’s very powerful, especially in combination with what Scrivener can do.
I think there’s a way to automate this from within Scrivener (see here), but I’m comfortable with the command line so I haven’t looked into that.