Hi Jakebodle. Hope you enjoy the forum and get as much from it as I have.
You are far from the first person to express the view that something is missing. If you read back to some of the earliest posts, and all the way through to your own, you will find that the "Secret" and the "missing 5%" is nothing new. The fact that you've done everything you already have and still feel the way you do is not surprising. I still feel that way most of the time.
There have been a few members here over the years with actual BIR industry experience and they've shared a lot of great information. There are some fantastic curry recipes here and in other places both written and in video. The is still no doubt in my mind that sometimes the curry you buy is just better than the one you make.
Secret ingredient, old spiced oil, singeing, tarka, banjara / bunjara paste, aluminium pans, high heat, etc etc. Nearly everybody has tried it all and occasionally lucks into a great dish, then if they're like me, usually can't remember exactly what they did. It must be pretty widely accepted by now that there is no secret and no single element will magically reproduce the dish you get at your favourite local. This comes from experience and knowledge in the industry and the mastery of spices, ingredients and techniques. You can still cook a good curry though.
I'm told the curries out here aren't anywhere near as good as proper BIR, but I think they're sometimes better than the dishes I've prepared using exact BIR methods. Even BIR is not one thing. Curry from different regions will be different just as different chefs will have different dishes. Unless you can get someone to actually show you exactly what they do from start to finish in the preparation of your required dish it will be very difficult to get close. Even then you may not get it exactly right every time. There is something missing and I don't know the answer. There are too many people saying it for there to be nothing in it. Of course there are some who say that they have cracked it. Without trying one of their dishes you'll never know.
CTM should be relatively easy to get an acceptable dish. I've always managed to produce tasty and reliably presentable dishes. Korma on the other hand, is a tricky one and you will find a great diversity in what different people expect of a Korma. Then of course there are some people who refuse to entertain the notion that a Korma is worth cooking at all.
Good luck with it and remember to let us all know when you've found the answer.