I'm 64 years old, and have been addicted to slalom since my early 30's. Like you, I'd sought out recommendations, visited instructors, read articles, and tried my best to practice everything I learned. Seemed each 'improvement' would help me for an hour or a day, but within a week or two my skiing would go right back - my videos would look a lot like yours. Early last year, everything changed for me - I started keeping my hips up easily, making my ski finish the turns smoothly, and my wake crossings came fast and stable. The change, for me, was remarkable. It was the first summer I've had where slalom was just plain fun, and wasn't either hammering my body, risking life and limb, or wearing myself completely out.
Here's what worked for me: Stand on flat ground in your slalom position - I'm RFF, so I stand with my right foot in front. Now, with most of your weight on your front foot and your front leg pretty straight, push your hips forward until your front hip is 'locked;' i.e., it's pretty much a straight line along the front of your thigh, your hip, and your stomach. This is where I decided my leading hip needed to be. Now, push your front hip back a little bit. You can feel it 'break' into the butt-back position I'd gotten so comfortable with. That's a position you never want to feel when you're skiing. I decided I would ski at all times with my front hip 'locked' into that forward position. I could tell if I had it right, because the change to the 'broken' butt back position is so noticeable.
I took that 'feeling' out onto the lake with me, away from the course. I focused on one thing only - keeping my front hip from breaking backwards. It was easier, on a ski, to push my front hip forward, and the 'break' from hip-forward to butt-back was more noticeable and easier to avoid. After my initial pull-out I noticed that, for my onside pull, as soon as I started crossing the wakes I was unconsciously letting my leading hip 'break' backwards - from that point, I could never get it back in the right place. So, I started refusing to let my lead hip 'break' - I kept it 'locked' all the way into and through my offside turn. It took some courage for me, since everything in me felt the the 'butt back' preturn was safer - but it's not. After a few tries, the 'new' hip position felt waaay safer and the turn was waaay smoother. It took quite a few sets to end my subconscious certainty that 'butt-back' was safer, but over a few weeks, keeping that hip pushed forward became a self-reinforcing habit because I was having so much more fun.
I'll admit this may work only for me; still, I'm sharing it because, for me, it was a game changer. Each of us knows deep down that it's not our lack of strength, desire or skill that keeps us bent at the waist, yet bent we are and bent we seem destined to stay. I've never pursued a sports-related improvement that looks so easily fixable, yet is so inexplicably elusive, so endlessly frustrating, and so ultimately demoralizing. It adds insult to injury that loads of skiers never seem to experience this problem - to them the fix is obvious and easy. I guess each of us needs that one idea that 'clicks' to get past this limitation. This was mine.
Even if it doesn't work for you, maybe it will bring you some hope that your answer's out there somewhere. If I can enjoy this kind of improvement after all my decades of struggle, you can too. Merry Christmas, and best wishes to the 'old guys' slalom community (and for that matter, to everybody else too) for a healthy, happy new year!