Thank you for your question's, concerns and history revolved around this crazy game we enjoy. To bring some comfort to your psyche, I meet around 5 to 10 new students a week who voice your same message on my lesson tee. You're not alone ...
Let me address the concerns of one "method", "mold" and or "philosophy"... I've been fortunate to study under some top 100 teachers prior to my becoming an instructor and I continue to do so. That being said, the best of the best don't have "ONE" specific mold.
To be honest, it's a possibility some of the prior instructors you've worked with were trying to study you and translate a pattern that works with your flaws and or misunderstandings (poor grip, setup, posture, tension, take away, pivot, transition, impact position...) and if not, shame on them ;). There's no one way that works for each and every individual. In my practices and experiences learning about the different personalities and biomechanics of students on my lesson tee, I find using traditional "feel" and "visual" demonstrations (whether hand's on, video, myself displaying, example's etc) is important so you can understand the "RIGHTS" & "WRONGS" / "CAUSE" & "EFFECT" in your challenges. This has been very successful for the students I work with in understanding how to create a better pattern in ball striking, shot making and feel around the greens.
When it comes to Video... Did the instructor specifically use Ernie Els as an example for where he wants the club position in take away, top or transition to be? Many times a teacher is using an exaggerated example to display what specifically you need or where the club needs to be. If it comes to the body, I myself usually try to match a body frame and age that matches the student at hand. It just must be specific to your understanding.
Playing worse before getting better? If I work with you on making change and developing a more consistent pattern, yes you may struggle a bit trying to build the correct pattern while working on the range at a half speed pace to earn it correctly within full speed on the course. Mind you, when you're playing JUST PLAY! The work and development we are accumulating on the range will show up on the course in time. Don't bring it there unless it's during an empty day where you can drop balls down on each hole to execute the new feel you're implementing.
Single Digit Handicap? Golf is one of those game's that never guarantees a great result. Why do we even play? Weren't we told growing up that if it hurts you, shy away from it and seek elsewhere? Ohh this crazy game. However you're in luck. Einstein said the definition of insanity is doing the same thing over and over and expecting different results. Well. Not true. True to some extent, but with golf it's only true if you're practicing with the wrong fundamentals and approach. Set yourself up with the correct formula and with committed practice there's no reason you can't be a single digit handicap. It just takes work. There's no secret. Sorry to be so honest. That's just how I've bettered players overall. HONESTY & HARD WORK.
All in all, it takes commitment and trust with whatever teacher you work with. Most teachers are laying a correct pattern and fundamentals for you to discover how talented you are. Then with time and some dedicated work practicing then implementing it on the course, you will reveal your abilities and how far you can go. I think your goals are realistic and Please feel free to ask and or send me video with questions and concerns to my email. My reward for teaching is hearing about the success of a player (no matter the level) achieving better scores and doing things they thought could not be done.
You can send me a PM at firstname.lastname@example.org Again thank you for your question and I wish you well out there on the course ;) - JJ