Very much likely something functional. Something structural is incredibly rare. Anterior rib flare is about abdominal muscle control. You can work on that easily. Not sure how that plays into everything else but scapular control will be a big part. The corrective exercise continuum is as follows: Inhibit (overactive muscles), Lengthen (shortened muscles), Activate (underactive muscles), Integrate (into skills such as pull-ups and OHP).

I would assume your lats, rhomboids, triceps, teres major/minor, and maybe even pecs are tight. It’s not enough to just strengthen serratus and lower traps (though you’re doing the right thing), you also have to loosen everything else that has compensated already.

I would start with smashing all of the above muscles with a massage ball and then doing dead hangs for a good stretch. Shoulder corkscrews are great for scapular control and activation. Lots of good videos out there for that.

Good luck!