Dragon/Tiger, Longevity Breathing POSTURE

I don’t have access to a certified instructor in person, so I need to ask on the forum…. I just got the book for Dragon and Tiger Medical Qigong as well as the video for Longevity breathing (the two appear to go hand in hand). LOTS of questions, so I’ll try to stick to the most fundamental…

I’ve been seeing a physiotherapist for years to (successfully) overcome common muscular and neuropathic issues that result from too many years of university studies, computer work and holding a musical instrument in front of my face. Essentially, I only have exaggerated issues that more and more people get these days from earning a living in a seated position with arms held in front. (Taxi drivers, accountants, receptionists, lawyers, computer engineers, website or game developers, etc.)

PLEASE CORRECT ME IF I’M MISTAKEN:
In Dragon and Tiger, the posture prescribed includes fully relaxed belly, dropped chest and shoulders rounded foreward…. these are precisely what I’ve been working to reverse! In my specific case, 30 years of incorrect breathing (over-exagerated belly breathing gave powerful music, but hibernated my ability to use sides and top of rib cage) I must ensure the transverse abdominis muscles are engaged and strong, sternum is well supported and the shoulder muscles strengthened and activated. Everything that affects the angle of the rib cage affects the curvature of the spine and keeping the arms in front so much for computer/office loosens the shoulders and causes neck strain.

My case is extreme, but I understand that to a lesser degree, the above is true for lots of people in modern city living. Would it be correct to assume that what is meant in the book is just full relaxation, right? More to avoid the carricature cartoon soldier standing at attention, but not ronsuggest slouching?