Csharpminor, I couldn't really get a clear picture of what you are trying to convey, nor how it is different or the same, since you cited me in your opening. My words are coming from experience and I hope you will let me share some of it with you. What I have written pertains to what works, rather than what I want.
The real goal is to be able to play the piano. If you are able to play the piano you can do all these other things: grade 8, repertoire you enjoy, etc. This is not as obvious as it may seem - the real goal is to be able to play the piano.
Now this ability to play the piano does not come from learning how to play this piece well, or that piece well, or even from learning to play your scales. It comes from smaller things that are behind all of that. These smaller things are .... not building blocks, no ...... They are like the parts of a car or an old fashioned wind-up watch. They all link together and work together. Each one must be the right shape, move well, and work together. If one of them is off, all of them can be off.
A good teacher is like a watchmaker, and I guess that makes you the watch-to-be.

The teacher knows how the parts fit together, in what order they should be assembled, and how to create some of those parts with snippers and a pair of pliers. Now, on the surface, what you see as a student, might be an assignment of scales and a particular piece. You are told to practice it, how to practice it, and the piece improves and you know your scales better.
But what is really happening, with a good teacher, is that what you've been told will start shaping those watch pieces and putting them into your place. If you are given music with that dotted note pattern, and told to count properly, and you're struggling with it --- that's a watch part being cut out of foil, and shaped. You're getting a sense of timing and rhythm into your mind, nervous system, and fingers. You will be able to focus more strongly than before, and your ability to play will improve. The
pieces you play will improve because you have new skills. Actually they improve for two reasons: the new skills, plus having practised the pieces.
Does it make sense so far?
If you understand this, then you want to get out of the way of the watchmaker. You want to allow this person to find the right thing for you at this time, and then the next and the next. You will pay attention to how he wants you to do it, what he wants you to stress in your practising, what things he is emphasizing. A choice of repertoire, and even a path such as grade levels in a system, will no longer be important. The grade level system is good because it gives a framework for acquiring these skills, by listing what needs to be learned and to which standards. It is nothing in and of itself, but it is a pathway and tool for getting there.
Teachers often hesitate to go all out in giving us what we actually need to progress, because on the surface some of what they will ask us to do may not make sense, or it's too hard, or we don't "enjoy it". If they cannot ask us to do what is necessary, then we have been shortchanged. If we insist on our own way, we cheat ourselves of some of what we might be able to reach. Knowing all this, I stressed what I did.
*** In regards to you teaching yourself. When you learn with a teacher, you must be an active participant, and that part of self-teaching can serve you. A student who passively does what he is told and hopes that by some magic he will improve will only go so far. For example, if you are told to improve your counting, and at home you set your metronome and let it tick away, it might not do much. If you say "1-2 and a -3" you might do one better. But if you give yourself the
intention of remaining in the beat, pay attention to what you are doing, listen to whether you are in time --- then you are actively engaged. This is what I mean by being an active participant.
If you change what you have been told to do, you may scew the outcome, or in some cases you might actualy find something that has worked for you --- but while you understand the process less, you might scew the outcome.
When you teach yourself, you may set your body up for habits that are hard to get rid of later. If you are accusomed to sitting in a way that is badly balanced, then later you won't get the power and nuances that you want. It is very hard to undo. It is much easier to acquire the right foundations in the first place.
KP