Just a small update and closure for this thread. I was finally able to ride a 56 and 58 back to back at a store 2hrs away. Those who said stack and reach are not independent of frame size were spot on. The 56 fit very well given my dimensions and despite the geometry chart indicating only a 4mm difference in reality it's a lot more of a stretch than that when moving to the 58.

Thanks again for everyone's feedback.