I have a stagger athletic stance, slightly bent knees. I don't know if it is because I am not leaning forward enough (that leaning forward + tactical turtle look looks kind of goofy to me) but I notice that the pistol is slightly pushiing my upper body back in shots. When I watch great shooters online I notice there is ZERO movement of their upper body or head, even in unusual one foot stances during competition.

So what am I doing wrong? I have the traditional thumbs forward grip. Super tight clamshell on the support hand. I do feel some movement in thee grip and have to readjust after I'd say every third shot which means I still am dialing in the grip. But what about my stance to prevent pushback? I am not a small guy either, 200 lbs so I KNOW it's my posture not being correct thats making me upper body get pushed back slightly after each shot.

Would love insight from ppl who have over time "figured" out the small details that made the breakthroughs

While I cannot provide a true answer to your question and while it appears you are practicing and researching, this link may be just too basic for you https://tacticalhyve.com/shooting-stances/.  After all, you already have seen and know what the tactical turtle is.

There are many styles and stances out there each with advantages and disadvantages to choose and practice with.  As for the upper body, it absorbs the recoil in all of them either through muscle or bone structure.  As for the videos of great shooters online, the two biggest variables compared to your situation could be the weapon they are using and the load as compared to what you are using.  The recoil alone from a reduced practice load is much less than a full defensive load.  I'm not sure if you are comparing apples to apples because those two items were not described in your post.  

It may be simply that you are leaning back on your heals as opposed to standing more on the ball of your feet so that the absorbed recoil feels more pronounced because you are slightly "off-balance".....

Good luck in finding your answer(s)!

Are you bent at the waist just a tad to put your shoulders forward?   In all honesty, you can separate the top half from the bottom half and you can shoot on the move with shoulders and hips not both being square to the target without rocking yourself back.  

Hell people shoot all the time from one foot using a hitch step while moving laterally and shooting directly down range.