I have 2 cents for you.
I have to first say that I am not a stack and tilt guy right now. But I am doing some self education.
Having said that, I have personally lowered my handicap every year. I have even dropped my handicap while having 2 kids being born. One is two and the other is now 7 months old.. The reason I feel is practicing the right things the right way. And I stopped going to the range everyday. I now go maybe once a month during the off season and maybe twice a month during.
Here is my process that helps me and I believe 100% that if you do it, or something similar, you WILL get better.
When working on big items I never go to the range. To many distractions and the urge to take full swings is just to much for me. Every ball I hit the wrong way is slowing down and hurting the learning process.
I do a lot of slow reps in front of a mirror at home. First without a club then with a club. It takes me many reps doing a certain move to gain a feel for it. Once I think I have a good feel (normally I have to to do near 100 reps) I will go out in the garage or in my yard and start hitting balls while recording some of the swings to see if I can perform the task while hitting balls. If not, back up and do more reps. This might seem tedious but it's really not as bad as it sounds. I will then only go to the range to check my ball flight or to gain confidence to take to the course. I used to hit a billion balls at the range and I improved VERY slowly. Basically what took me months to improve, if I even made an improvement at all; now takes me a week maybe two for small things while some might still take a couple months.
Its nearly impossible to make a full swing at speed while trying to hit a ball a certain way and make quick improvements. So if this is you.....quit it.
I started from scratch and worked on the setup and grip, then takeaway,then back swing, and so on and so on. Every new piece I make sure I am still doing all the other preceding moves correctly. Practicing to many things at once is a recipe for disaster for me. I play well thinking about one thing at a time. Any more than that I hit the ball terribly.
I know you practice a lot. But I am not sure how much practice you put in with just the individual pieces?
Hopefully some of this helps you.
My swing advice, once you get to the top of your back swing in a proper position( whatever that might be for a stack and tilt swinger) I would figure out what is causing you to lose your tush line during your downswing. I had that move and guess what I hit? blocks,shanks, hooks, fades, thin shots,chunks. Did I leave any shot out? :)