Anyway all I have is my own personal experiences with anxiety, and I can at least confirm that breathing plays a huge role in mood regulation along with physical posture, staying hydrated, and gut health.
Bad posture causes muscle tension making it hard to relax. A massage gun to the neck, shoulders, and/or back has calmed my panic attacks very effectively before. I discovered this on a long road trip years ago.
For gut health it really depends on what your forms of indigestion are. Common ones are lactose intolerance, not enough fiber, acid reflux, etc. Even just overeating can trigger anxiety since heart rate goes up and it causes weird sensations. Dehydration has a strong effect on your heart rate and blood pressure, and alcohol can also cause nutritional deficiencies in unintuitive ways.
These all sound silly until they happen along with some other external form of stress and it all piles up at once. I think just about anyone would spiral into a panic attack if the list of discomforts becomes severe enough and for long enough. Everyone has a breaking point.
Anyway I think the more interesting topic is stress management. Living deliberately and being organized is probably far more effective than trying to "control" your fight or flight instincts. Of all the things I've ever tried, performing breathing techniques while freaking out makes anxiety so much worse. I'm better off avoiding things that fuck with my breathing enough to cause me to think I need to manually intervene in the first place.
Yes; one of the first things my therapist showed me after just talking about stuff was a breathing exercise, with the first thing being assuming a position that allows for easier breathing (lying down flat, relaxed, but feet propped up / knees in the air). "Relaxed" breathing is deep and slow breathing through the stomach, but you can't do that well if your posture is off and I can imagine this has long-term effects. Subtle, of course, and it's never one thing that causes anxiety etc.