My suggestion to OP would be to study the works of the great photographs who devoted their work to photo documentary. Dorothea Lange, W. Eugene Smith, Eugene Richards, Dany Lyon, Josef Koudelka, Gordon Parks, Susan Meiselas, John Davidson, Robert Adams, etc., all deeply cared about their subjects. No great photographic documentary was ever done by photographs who were "neutral". On the contrary.
In fact, the first thing they teach you in journalism school is that "neutrality" is a non-existant concept in journalism. There is no such thing. What we talk about is balance, fairness, impartiality, integrity, indepence and accuracy. In other words, a full commitment to investigating and telling the truth. The very foundation of this has nothing to do with being unbiased of "neutral". It has to do with how rigorous you are with your methods of investigation—i.e., fact checking—, with being able to understand the difference between fact and context ("Facts are neutral, context isn't", as the journalistic saying goes) and know when which is more important and how one influences the other.
You are not disengaged. And opinions are fine, as long as they are informed, as long as they are based on the same rigorous methods of investigation. In journalism, commitment to the truth, as Kovach and Rosenstiel stated, is not only "getting the facts straight" but also "making sense of the facts." Only thing is you need to get the first part right — opinions based on misinformation, purposely or unintentionally, will only lead away from the truth.
In general, the reason you will embark on a documentary project is because you care. This means you have a bias, one way or the other. Doesn't matter. On the contrary, if you don't care, chance are you will only make ordinary, boring photos. Every great journalist, and photojournalist, knows that the first thing you do is find your angle—what is the story about, what's important or interesting about it, and how am I going to tell it. You can't find an angle if you don't care, if you believe you have to be "neutral".
Gene Smith deeply cared about the what happened to the people in Minamata, Eugene Richards deeply cares about poverty in America, Gordon Parks deeply cared about segregation in the south, Koudelka cares about the lives of the Gypsies, Robert Adams cares about the environment, Dany Lyon, well, he just cares. Examples abound. All these photographers were all faced with a reality and chose the photographic method to tell the truth about it. What they all did is start by sticking to the facts and trying to make sense out of it: this is what is happening in front of me, what does it mean, how is it meaningful.
Remember, facts are neutral, you don't have to be. This is why the camera is in many ways an easier tool to work with than words. Stick to the facts, and that helps you not taking sides. Again, not taking sides doesn't mean being unbiased or uninvolved: most, if not all, great war photographers are always incensed by the idea of war; their indignation is often their motivation; they often hope that showing the absurdity and suffering inflicted by war will change people's conscience of it (this did indeed happen with Vietnam in the 60s). This doesn't mean they take sides. They aren't saying one side is good, the other one evil. They are just stating the facts, and helping viewers make sense out of it.