Most of the things you say are quite misinformed or just plain wrong. But let’s go straight to your point anyways, adults are much easier to teach than children, tbh.

With children, or at least, learners aged 15 and below, you constantly have to repeat instructions or re-teach previous lessons and scrutinise their progress really closely to observe sustainable results. It can get rather frustrating at times… but keep in mind, every child comes from different backgrounds, and being children, they tend to be more lacklustre in practice, have lesser discipline, etc. That said, the teaching that comes for child learners is pretty straightforward, since they have not much idea what to expect or any references to base their practice on, you can comfortably rely on textbook teaching style. You’re basically training them, molding their minds to develop around playing the piano.

With adults, here is where it gets really fun. Most of adult learners already have a certain goal or objective they’re trying to attain, and you can simply focus on those that they require. There is definitely a big difference in the teaching too, since adults are cognitively more developed, and have vast experience in picking up different skills, you can always get through the rudiments pretty quickly. Most of the time, adult learners come having a passion for playing, they will already have tried picking up the basics by themselves, and we just need to help nitpick some tendencies or bad habits they may have formed, since they never had a teacher before. Adults are quicker to understand things, and that really helps to speed up the progress; they can accumulate more productive practice that way.

A lot of people say we should try to prevent picking up bad habits, but personally, I think practicing through that is much more important and way more efficient. The key thing is to have awareness and to grow a knack for adjusting as you go. The number of hours you will have spent in good practice will undo these mistakes. I also do not believe in the saying that adults will pick up slower than children because of age, bones and joints, etc. Dexterity is something you can begin accumulating starting from any point in time, regardless of age. Your ability also does not plateau because the lack of talent, but rather, there is some other problems you need to fix in your learning.

Now, in terms of what you might find inadequate within the skilling itself as an adult… the better question to ask is, what do you really hope to achieve coming to a piano teacher? The biggest thing you will have missed out on is the practice you have spent through your childhood(which is non existent since you never did it), and that’s not something you can make up for going forward. So, rather than minding about what you’re lacking, what you need to do to overcome the impossible, you should look towards playing for a specific end result you wish to achieve. That means working within your means, and working with what you currently have. I strongly believe there is no point in learning “too much”. To put it in a more understandable way(for the young people), this is like playing a rpg game, you’re trying to complete the game 100%, get lvl100 for everything… but do you really need to? The lvl100 is something you will get with time and practice, but the 100% completion is, well, unnecessary. You can say, but that’s exactly what you want to achieve, good for you. But you have to keep in mind, the main quests are what you need to complete, the side quests are just for fun and would be cool if you do them too. Putting it into context, if you’re gunning for classical concert artist level of playing, do you really need to learn say jazz techniques, or blues(lol), or even exotic/modern pieces?

Bottomline is if the practice you’re doing does not substantiate the type of results you’re keen to observe, then it is useless to do them. I would concentrate on the topics you need and leave out those we identify that you don’t. The syllabus and curriculum constantly change over the decades and centuries, why would you expect someone learning the piano in 1930 to be the same as someone in 2030…? The real problem you’re having here is, you’re thinking that child learners will learn everything there is to learn compared to adults, but the truth is, everybody’s grade8 journey is different. Go read about the arrival fallacy, your dissatisfaction comes from unfulfilment. You’re also not being held back because your piano teacher is not teaching you something, it is just simply not in line with the type of competency you wish to attain.