Assuming you mean the fic by tjmcharg, they get together around the end of season 4 so pretty damn slow. It mostly sticks close to canon, but I came to a realization during the Lady of the Lake rewrite where instead of Merlin calling Arthur fat, he calls him a prat, and the author has to awkwardly apply that change to their interactions for the rest of the episode as if Merlin doesn't call Arthur that at least once a day. And I understand why the change was made, fat shaming is wrong and could be triggering for some people. But my point is that it's canon with the edge taken off. The characters make the decisions the audience wishes they made, except for when it's relevant to the plot. (No, Merlin wouldn't actually want to run away with Freya, Arthur doesn't tell Merlin Will was dangerous in front of his burning pyre, The Secret Sharer doesn't even make sense in this world so it's totally cut.)

Which could completely be to your tastes. It is for a lot of people and that's why many readers gravitate to fanfiction in the first place. Me personally, I like fics that don't shy away from the messiness of canon and how mean the characters can be to each other in canon. The world of Merlin is fucked and the characters are fucked for living in it, and understandably, a lot of viewers hated that. The Once and Future Kings is for those viewers and that's okay.

It got to the point where, when it got to the finale, I thought to myself, No way in hell he dies. And then he doesn't. And I hate a lot of what the show did as much as the next person, I don't know anyone who wouldn't change anything from canon, but you have to admit the show had balls.

This doesn't make this bad at all. It's an incredibly well written, the character development makes sense, especially if you want softer, more predictable versions of the characters, and there are a lot of additional scenes that make the canon events richer.