I think we had a language/communication issue.
You said the 2nd photo was “right” and that you had to actively concentrate on your serratus and to focus to hold the correct position. I was agreeing with you.
If you had this shoulder problem for awhile, it’s possible you have other posture issue. Maybe one shoulder or hip is higher or lower than the other. Or one rib is flared and your torso is slightly twisted one way or the other. Maybe you are slightly crooked, but you are so accustomed to it you can’t even feel it.
When you do shrugs, standing, with both arms working at once you can feel it. One doesn’t line up right and it’s hard to use the tarp to shrug.
I would try to do the shrug Seated on a Bench, One arm at a time. Focus on symmetry and each arm working the same way. Sitting down, it should be easier to feel and adjust for any posture issues. In my opinion it will be easier to get the bad shoulder moving better if you can focus just on one trap at a time.
The pallof press is also good to challenge posture and back position.
You could also do face pulls, 1 arm rows, shrugs or overhead press in the half kneeling position to challenge hips and posture if you have similar issues with those lifts.