Yami no Bansosha is not really about a mangaka, but one of its main character is a manga editor (and features a suspect that might or might not be a manga artist), and it talks a lot about manga editorial/publishing process.
And if you haven't watched it already, I strongly recommend
The Kingdom of Dream and Madness, a documentary about the famous Studio Ghibli, which gives you a small insight of how an animation studio works.