The main issue is that this is supposed to be the turning point for the Union, when they finally get their stuff together and turn the tide against the Confederates, but it feels less as them winning than the South losing. They didn't win because they had a good plan and took advantage of any enemy mistakes in the heat of battle, they won because the CSA command was basically out of commission (maybe that's a little too exaggerated, but you know what I mean) and made irrational decisions.I understand if this doesn't feel like a great narrative moment, and that's because life often doesn't make narrative sense. Victories are often the result of outside factors or coincidences. I do think the victory is the fault of the Confederates' hubris, which has been bolstered by 3 previous triumphs (Peninsula, Bull Run, Frederick). In this case, I think the victory disease is somewhat justified because when Lee took the reins, everything had been going wrong for the Confederacy in the East. They had lost the First and Second Maryland Campaigns and Anacostia, and the enemy was at the gates of Richmond. Enter Lee and not only he turned the tide, he destroyed the enemy. Up to Union Mills it seemed that indeed he was capable of doing anything, for Hooker failed at Bull Run and Reynolds at Frederick. This hubris was also shown in how they thought the Black troops were useless and would simply break. It also responds to factors outside the story, in this case, that I partially wrote myself into a corner - another big Confederate victory or just a semi-triumph like OTL's Antietam would have turned this into a Confederate victory TL.
I mean, the Peninsula Disaster is already kind of like that - Lee destroyed two corps but the Army of the Susquehanna was back on the field very soon. Another similar moment may take place in the future, when trench warfare starts.
Yeah, the Peninsula was rough, but I could see it as a nice contrast to it. Where in the Peninsula they broke and took a long time to get men and their morale back up, in this North Anna they keep on going, quickly making up their loses with volunteers/black troops. It would show how far the army's come