How am I supposed to know what YOU need to improve? Use your own head. Trust yourself. You know your weak spots anyway: dare looking at them.
Okay I am just like you in terms of slow improvements, so I just tell you what I hope works for me. So far it's looking good on my end.
I tried to identify my weaknesses for ages now. I've always known my mechanics suck. I can pull of a combo but can't quickly react with a flexible answer to a sudden problem, which is a big deal. Hard to train - knowing that did not help me at all.
I am bad at defensive play & playing from behind. Okay identifying this helped a bit. Some wave management stuff was quite possible and helpful to learn. Not exactly a breakthrough but still a gain.
Well this could go on and on with many minor weaknesses. I reckon "the real" reason is twofold. For one thing I do not take in information that quickly, which is a hard wired brain related issue I think. Mind I am way quicker than most when taking in complex Information, as in math or whatever, but these split second things? Epic fail. No tactical overview. Even got Yolo mouse to not lose my own cursor... The only way I found for this to be tackled was by developing automatic routines. Like Kung Fu: repeat the same reaction 1000 times and you'll do it immediately and perfectly the 1001th time, w/o thinking about it at all. Takes forever because there's no sandbox and no drills.
The other is the opposite in a way: I play autopilot. That is a major weakness. I just said that I believe that crucial weakness of mine to only be solvable by repetition and automization, yet now I say: I automize to the point of not using my head anymore, not getting the big picture. THAT is what I believe to be the No1 weakness among most if not all low elo players. You cannot even call that weak decision making: it is not making decisions at all, going full autopilot.
I tried to tackle this by picking a supposedly weak off-meta pick, as in: make good decisions or die trying. 200 Lb sup games later I learned a lot about lane dynamics, powerspikes, mid-late support play and others. However: I became good at it. Great, right? Not entirely. While it has become a nice pick that is immensely fun and actually effective it is no longer the best pick for learning how to make decisions. The big question mark, "how do I make this work?", vanished. The autopilot is back...
So I picked up Soraka lately. I've always been bad at peeling. I can tell you: it helps. In just a few games I learned a ton. My conclusion at this point: break your habits. Try something else entirely, ditch the play-to-win nonsense and dedicate yourself to improving. Stick to something that requires you to learn new stuff and make a conscious effort to do so. The unusual setups lead to more conscious decisions concerning questions you never even needed to ask before, only now becoming relevant. Play it extensively to really solve the questions, master not only the champ but the general problems that came with it. Ditch it as soon as you feel really comfortable on it, as soon as you stop asking questions. Mind this can relate to general stuff, e.g. your answer to "how do I support with Jinx?" could be: "mastering the trading stance" or "superior wave management in mid-late". Since side lanes are ignored at that elo anyway you can as well manage them as a support, right? You can do the same thing with Morgana sup of course. Only that you'll most likely end up not answering that general question, rather just finding a way to be successful with good shields and skillshots. That's even if you try to make the general question your thing. Nothing wrong with learning to shield properly though.
Sad news: no matter how you look at it - it isn't going to let you improve super-fast. It only breaks a stand-still and possibly picks up a half decent pace.