Shirt fitting - I need some advice regarding how I should remove the excess flare at the lower centre back seam?

I apologize that it is not a jacket. I hope that I can still make this post in these boards.

I am trying to create a brand that flatters a muscular male physique. Below you will see what our brands size large dress shirt would look like. I am slightly larger than this but I am wearing the shirt to illustrate my dilemma.

Creating a fitted and flattering shirt has proven to be difficult. I am happy with the fit of the overall shirt except for the lower back of the shirt. Muscular guys have very developed and rounded upper backs and a hollow lower back. My pattern has been shaped at the centre back to accommodate the extra room needed at the upper back and tapers in at the lower back. However, the adjustments made to the pattern to create a taper at the waist now creates a flare. I would like to remove this excess flare.

My question is; how should I go about removing this excess flare?

Should I take in the amount of the excess at the lower back and add it to the side seam?

Is it flaring out because the shaping at the centre back is too extreme that I should release a bit at the waist level on the centre back? or is there another way?

Thank you.

https://www.youtube.com/shorts/fuLVXQ9pohc