Makefile-driven development. Run "make pdf" as needed (looped in a shell one-liner if you prefer, or driven by an event watcher). A decent PDF viewer will either reload the document automatically on change or can be readily reloaded. The Suckless PDF viewer zathura is among the former, I've also used, variously, xpdf (slightly grungy these days but an old reliable) or MacOS's Viewer app.
This lets you work on the doc in a terminal window and have the (reasonably constantly updated) formatted output in a PDF viewer.
Short documents will render virtually instantly. I've not had long renders until documents extend to at least several chapters worth of text if not book-length, and even then it's a matter of a few seconds in most cases. Highly-formatted texts may of course take longer.
Why should you have to make a PDF? It would be even easier to have a live Markdown-editing window with preview... or better, just a WYSIWYG Markdown editor.
You can substitute whatever output endpoint you'd prefer, e.g., ePub, HTML, etc., if PDFs don't satisfy your personal itch.
I happen to generally be aiming for PDF output, it's among the more complex to produce, and still runs pretty much without concern even on very large documents. Anything else would be even more trivial.
<https://pwmt.org/projects/zathura/>
This lets you work on the doc in a terminal window and have the (reasonably constantly updated) formatted output in a PDF viewer.
Short documents will render virtually instantly. I've not had long renders until documents extend to at least several chapters worth of text if not book-length, and even then it's a matter of a few seconds in most cases. Highly-formatted texts may of course take longer.