For all markdown docs now I've moved to just using VS Code with a live reload UI, the best DevUX I've found is VitePress [1] which immediately updates on save and shows the real thing, i.e. exactly how the docs will look including rendering any custom markdown-it extensions or Vue Components embedded in the page.
One free Markdown editor I have started using is Notable [2] which has become a worthy replacement to Notepad for manually managing TODO notes & sporadic text files in a neat simple UI with built-in search, pin working docs and custom labels for quick organization and retrieval. It works exactly how I wanted it to as a minimal UI for editing static .md files in a directory with all metadata stored in frontmatter that displays preview mode by default and an unobtrusive text mode when editing it.