Indeed, I skimmed a bit and misread “unable to break” to mean that you wanted a line-break opportunity but the renderer didn’t allow for it when a letter is directly following an em dash. But it’s the other way around, you want a line break in the source after an em dash to not translate into a space in the rendering. This would likewise be possible to handle by regex replacement as a pre-rendering step.

More generally, I see markup languages and the details of how they are rendered as largely orthogonal. You don’t necessarily need to invent a different markup language in order to adjust the rendering.