Posture is not a pose
Sitting up straight for ten minutes and slumping for eight hours is not a posture strategy.
People hold a perfect posture the way they hold their breath: briefly, with effort, and then collapse. The body does not need a correct position so much as frequent changes of position.
The best posture is the next one. Shift, stand, lean back, walk to fill a glass of water. Movement spread through the day does more than any single correct shape.
When a client asks how to sit, the honest answer is: differently, often.