Lots of good suggestions.
I definitely believe holding calm in the water makes a big difference! I'm fairly good at the trim and no extra motion thing. But other things add up as well.
Exertion, and cold, makes a difference. Three dives in Belize, one holding-not-dodging against current that canceled the next dive .53 SAC; two drifting with current, .33 SAC. Four dives in cold NorCal, two hovering over reef .4 SAC; two coasting along a pipe .55 SAC.
Being short of breath on land is not healthy for you, and is unlikely to help your SAC underwater. Fitness will help your diving by being better for you. It seems like yoga fitness and calm would help scuba, and health. I've done high level yoga in the past. Fitness helps on the pre-dive surface swims.