I would remove the upper and lower bicep return edge first then see how much space you have remaining between your body and the armor, I like to use the 1/2" rule for the space. If afterwards you've increased the space then trim more at the butt joint edges, closer to the 15mm goal. If there's no change then still stick with the your current armor measurement yet still install 15mm cover strips. Tape them in place and post a photo here.
You appear to have broad shoulders also. I encourage you to remove a good majority of the shoulder bell lower return edge. This will bring the bell inward towards your chest reducing the bell to chest plate gap Sly and myself most commonly suggest fixing about during L2 approvals.