+John C You mention my background, which makes these questions much more challenging than they seem on surface. One phrase I like, "posture is a relationship, not a position". So the right posture can look different on different bodies. The key is getting the right muscles prepped while aligning the geometry for a good path. I think it's easy to make general statements ("straight spine, more rounded, athletic") but practically they are very limited.
If I had to come up with a simple rule, I prefer posture to be on the flexion side of neutral. Neutral is roughly halfway between the range of non-painful movement.
As far as the Rotary Swing guides you set up, I'm not a huge fan of excessive lat activation. I think many golfers who get steep use them too much in transition. But I do look for core/glute engagement. I often test my students to make sure they can use those muscles. If they can't, then forcing a "proper" posture is typically not the most beneficial.
For your last statement (going from rest to active). All tissue has a threshold, if you surpass that threshold, then the tissue is damaged. Rapid speeds can surpass threshold, and the threshold at end range of motion is typically weakest (most susceptible to failure). So in that sense, I do like a ready state as part of the set up. In my in person certification, we spend time helping golfers feel core engagement at set up with something similar to this - https://golfsmartacademy.com/golf-instruction/feeling-one-piece-takeaway/