Gettysburg is overrated as being a the turning point of the war. Just Saying.
I imagine we'd agree on very little w/re: the Civil War, but this is absolutely true. Even if Meade fails at Gettysburg, Washington would not have fallen (the federal defenses there were massive), and Grant takes Vicksburg regardless the next day.
I've argued here before that any alt-history with CSA winning has to have Antietam as a decisive Confederate victory as its starting point. That's about the Confederacy's best case scenario imaginable because it's early enough in the war that a lot of the disasters of 1863 for the CSA can be avoided.
By Gettysburg, having the South win individual battles just prolongs the war, because, yeah, Vicksburg meant the USA had complete control of the Mississippi River plus complete naval superiority and the war became just waiting out the inevitable Southern starvation, in the best case for the South.
I think the more feasible path to a Confederate win would have had to have been if Donald Fairfax followed Charles Wilkes orders to take the RMS Trent as a prize, which would likely have pushed Britain into the war, and possibly France as well. A stronger Confederate performance during Antietam likely prolongs the war, but fails to change the outcome.