Maybe I've got it wrong but I always thought Lee should have just quit Gettysburg once it was established that the Union held the more favorable high ground. Just keep marching on Washington until he was able to secure the ground of his choosing. Sooner or later the Union army would have had to chase him down. That would have made him the great general he was purported to be.
Lee should have transferred his Army, or as much of his Army as possible, to save Vicksburg. The Confederacy was basically dead once they lost Vicksburg. Once the North had split the South in half, prevented imports to the coming up the Mississippi by occupying New Orleans, and Southern exports from leaving New Orleans, it was over. Sure things made it west to east across the Mississippi, but not as much as before. Southern commerce was staked through the heart. Union gunboats would blow any Southern supply attempt out of water. It is hard to find a more definitive Strategic victory for the North in the entire Civil War.
Instead, Lee thought he would raise trouble by invading the North. He did, scared a lot of Northerners. But unless he completely destroyed the Army of the Potomac, messing around in Pennsylvania did not matter much in the big scheme of things.
Beating the Confederacy was more than just winning battles. It was about destroying the ability of the Confederacy to feed itself, to arm itself, to wage war. Sherman understood this, and made Georgia howl. Civilians in a 60 mile swath suddenly found themselves burnt out, no food, no animals, no transport, and a burden on the rest of confederate society. Eventually, the system would collapse, and it did.