Syntax highlighting for markdown input is a good idea. I too have a markdown input and i have two modes for render - either the github style where you manually toggle between editing and rendering or i have two columns with editing on one side and live render on the other.

I too was close to making my own wysiwyg, trello for example has a md wysiwyg, so i knew it was doable with contenteditabe. But after talking with the dev community I was constantly warned by people who took the path before to not do it. So in the end i did not as i did not want to invest more time than i wanted to commit to the project.

Good for you, that you did and you made it to the finish line.