Focuswriter used to have a version available for the mac. It seems missing the last year. I'm not sure if that happens because Graeme has no mac computer to build it.
There's also a sister app called
Ghostwriter (not from the same author, but sharing the same aesthetics and part of the theme manager) that's focused on markdown.
It also lacks a version for mac. Both apps work fine on macOS as long as you built them from source and, since they're qt apps, you don't need XCode to build them, just brew and qt from brew.