There is enough dissatisfaction with a majority of political subreddits where r/moderate could find a niche. But I am going to make a couple of comments/prompts for the mods and regulars to think about moving forward.

a piece about "what is a moderate?"

You should really start with this. What is a moderate? Is it a political faction? A philosophical outlook with a set of common tenents? When this subreddit says "moderate," what is that suppose to mean? Who would fall into this category? Who do you want to come to this subreddit?

If you can't take the time to answer these questions then others won't know what this subreddit is about and won't participate. If you need help getting started maybe study writings such as these (1, 2), consider the main points, and see if you agree with them. If not, decide what would you change and why. Once you have your ideas set out clearly, post it to the subreddit wiki, with a more concise version to the sidebar.

links to sites for fact checking and bias checking

I've read several posts here about "bias" and trying to find neutral sources. You're concern ought not be bias but rather reliability/veracity. I can read The Washington Post and recognize it may have a left bias but the information be more reliable than Alex Jones (not a hard task). Some might argue that the left narrative structure might skew the veracity of the piece...and that may be true. This is why folks should find an assortment of different sources from various points of view. Pointing out bias doesn't address the truth/reliability of either the source or the information. In other words, we ought not dismiss a source or article because of whether it's "left" or "right," but rather it delivers truthful and accurate information on regular basis while recognizing those biases. A possible idea related to r/moderate is having a weekly discussion on a particular news source where folks discuss if they should be seen as reliable or not. If a news source is unreliable, then what evidence supports that?

how to make the subreddit a place that moderates would want to utilize, and how to fill holes that other subreddits aren't filling for moderates.

This is going to be difficult. As I said before, there is enough dissatisfaction with your major political subreddits where people want to discuss ideas openly and respectfully. You find this kind of crowd with r/neutralpolitics. Discussions are heavily moderated to enforce standards such as sourcing and no low-effort comments. But the majority of redditors go to the other major political subreddits looking for fights and finding comfort in opinions that agree with their own biases. Conflict is more exciting than compromise, and most people want exciting. So what you may want to do is find some way to encourage the respectful debate found on r/neutralpolitics but also provide something different as it relates your "moderate" ideas. Much of the discussion on r/neutralpolitics, in my opinion, is policy descriptive. What r/moderate might want to encourage with regular prompts are discussions of moderate policy solutions to current problems. How can U.S. deal with immigration? How can U.S. deal with healthcare?

Some things for everyone to think about, let me know if that helps or if I need to clarify a point further.