Ok, well, taking that purpose as a point of departure, I think you can reason out details like clothing etc.
One concern you seem to have had was/is how people will respond to knowing they'll be photographed. You also mentioned that the protesters you photographed earlier might have been concerned about the images appearing online. If your intent is to inform the world, and at the same time you don't want to cross your subject, you're in a catch-22. You either have to listen to yourself and follow through on your goal, or pay heed the desire of at least some of your subjects to not be photographed/published.
Of course, in practice it isn't as black & white as that. You're not dealing with one subject, but with many, and they may all feel differently. So you may find yourself explaining patiently to some what you intend to do, while refraining from photographing others and happily shooting away with yet others who don't mind.
Since your goal seems to be intrinsically motivated, from your own convictions/desires, in terms of dressing up, I'd dress as...you. No need to pretend anything. If you wear a suit and tie all the time, wear that. If you wear jeans and a black T, then that's it. If you prefer to appear in a dress with flowers and an extravagant hat, then why not wear that. In the end, it'll be the most consistent in this project if you present yourself as genuine as possible. That your appearance, your gear or just the mere fact that you bring a camera to your eye will provoke a response from the subject - I'm afraid you'll have to live with that. The only way to prevent it, is to try and sneak your shots unseen, but this is (1) usually quite challenging especially if you want/need to get close and (2) ethically dubious.
You're probably not going to like this, but sorry, there's no such thing as "neutral" and free of a "political agenda". You'll choose subject matter based on your convictions, and there's not a damned thing you're ever going to do about it. Striving for neutrality in reporting as such is a conviction just the same. So even if you manage that (which you won't!), it'll still not be neutral. I'd move past this and approach it differently: be honest (especially to yourself) about your motivations, your agenda (there's always an agenda, even if it boils down to showing beauty, promoting yourself, learning something etc.), your ulterior motives etc. Once you're clear about that, you can also much more consciously choose what you'd like to avoid. As long as you don't choose, you're stuck with having to navigate the ethical challenges as they arise. And that invariably means you end up being inconsistent. Having seen a bit of you here on the forum, I think you're far too conscious and serious to like doing it that way.
Nobody's neutral. And hey - that's OK.
I both a agree and disagree with this. Let me see if I can explain by example.
I don't care much at all about sports. I don't care who plays, who wins, who the coaches, and so forth. I am "neutral" because ... I am indifferent to the whole business.
But I deeply dislike the culture that surrounds Big Sports - the bad behavior of the fans, the multimillionaire (and billionaire) owners that bilk the taxpayer out of money to pay for their private stadium playgrounds, the personal excesses of the players, the bad manners exhibited on field and so forth are all things that I find pretty offensive. I cannot possibly be "neutral" about those things because ... I care about them.
But what I can be is
fair. I can make sure that I take into account my own biases, I can consider the fact that many other people disagree with me and find great value in such events, and the the players and staff are there to do their jobs, and fans are there to have a good time. In doing so, I am able to fairly and conscientiously portray what is happening at such an event and thereby do it photographic justice.
I spent a brief part of my early years doing wedding photography on- and off. Nothing will make you hate the bridal party, the groomsmen, and the families more than shooting weddings. It's a stressful day for all involved and the resulting behaviors are sometimes pretty awful. But, if photographers couldn't put their own views aside, no one could be a wedding photographer (except, perhaps, for masochists
The only time this becomes an issue is when the photographer (or writer, or TV talking head, or columnist or ...)
decide to insert themselves into the story, however subtly. The past 20 years or so have seen this noxious trend where people who are supposed to be describing what they see, are doing so through the filters of their own experience, beliefs and biases. That is their privilege, but that is not
reportage', it's just agenda pimping, otherwise known as "propaganda". What is most noxious about this isn't that they have a point of view, but rather that they pretend their opinions are the truth.
So I wouldn't say that the goal of a documentary photographer is the be neutral. As you point out, it's nigh on impossible to be so. The goal is to faithful and fair to the story, preserving as much context as possible, and getting yourself out of the way to the degree possible.
Don't get me started on the latest form of currently popular mental cancer known as "My Truth" that is yet another kick in the head of reason and decency ...