I would like to start off by saying that I agree, this won't be a full-featured wiki, and it requires an admin or mod to add certain people to a certain group. But, other than that, I want to let you know that you can create wiki posts with Flarum core. No extension needed.
The way this is done is by utilizing a tag, and permissions based on that tag. You can create a tag called "wiki." Then go to the permissions settings and create a group. Name this new group whatever you want. Wiki, or Contributors, or Gurus, or Wiki-folk...
Next, add a tag based permission. Flarum is very intuitive and easy to use and so I'm not giving extremely detailed steps here. But, I think you will figure it out easily. After adding this new permissions column, set it up how you'd like. You can go crazy and let any member edit posts tagged wiki. But, that's not recommended because of all the crazy trolls lurking out there. Most likely what you should do is set up the permissions so that only people in the new group you created can edit posts tagged wiki.
You can play around with different permissions settings, but I also recommend to make it so that only an admin can reply to posts tagged "wiki." Why? You mean there will be no replies? No discussion? Are you insane!? Yes, I'm insane, but besides that the reason is: You may have your global permissions set up so that posts can't be edited after 10 minutes. Or, after one reply. Well, that will mess up the wiki-like experience. So, to ensure posts tagged "wiki" are editable forever, simply don't allow replies (just for posts tagged wiki), and make sure your main global editing permission is, "until next reply." OR, if you are good with your global editing permission being, allow editing indefinitely, then you can allow replies on these types of posts.
That's about it. It is extremely easy to set up. All that is left to do is add people you trust to your new group, and explain to them that if they tag a post with "wiki," anyone else in this new wiki contributor group will be able to edit their post. Effectively making any post tagged "wiki," a wiki.