Greetings Roger,
Teaching children vs adults is a very particular interest of mine. Your discipline at the piano is to be admired and you would knock the socks of most of the youngsters no matter what talent. Most children simply cannot practice daily because they have no personal motivation to do so, most children treat playing music as a type of complicated game and only usually practice to satisfy their parents/teacher. If you can apply yourself every day without failure then you can only eventually win, discipline is the number one driving force behind all successful people.
I like your repertoire that you have learned, very good music to develop with and you have done quite a lot in a short period of time, it is certainly a result of your consistent effort.
Children tend to memorize music more readily and accept what their hands do without thinking too much compared to adults. Adults have preconceived ideas, they take notice of where they are every single baby step of the way to the solution, generally a child does not take note of their progress as closely. Children tend to have less conscious thought attached to their muscular and sound memory while playing where adults tend to think a lot more about what they are doing while they are doing it.
Adults however have experience in many life skills which aid their piano learning and easily trumps the child's natural ability to accept music. A child also is learning new muscular memory actions outside of the piano world constantly as well. Things like opening a door, riding a bike, learning to write with a pen etc, their brains are constantly absorbing new muscular movements all the time, thus when they go into piano the brain tends to accept all these new movements without too much direct thought. Their brains at an early stage are a clean slate and has plenty of room to add new muscular movements. It would not hurt for the adult student to learn new muscular movements outside of piano to get the brain cogs turning.
The adult also has a huge amount of space in their brains to learn new muscular movements however sometimes because you have existed for so long without having to learn any new muscular movements your brain can be rusty and not want to learn them without a lot of effort. It does not however effect you if you have a disciplined approach to your work, eventually the brain gets on board with constant efforts (like you say you do 3 hours a day every day which is a absolutely fantastic habit!).
Probably the hardest thing to teach an adult is coordination. I have taught some adult students in their mid 60s who could not even tap twice with one hand vs once in the other. Some cant even pat their head and rub their stomach and then switch. These issues are terribly difficult to instill if the adult student has no concept. The same applies for rhythm and beat, very easy to teach children but if the adult has no idea about this it is a very difficult struggle.
I find when teaching adults I am explaining a great deal more in their own logical terms where children merely accept. I think an adult needs to learn when to merely accept and when to think. One thing about learning piano is that you cannot control everything you do, you cannot force your brain to memorize a passage if it doesn't want to (even if you apply countless hours and try to force it you may make some grounds but it is inefficient). Adults like to think that everything can be measured and controlled, the paradox is that sometimes not trying to control an issue and merely accepting you will improve, can act as a catalyst to the improvement of your overall pianistic development. Learning to move on when something is not completely perfected is something that most adults have a tough time trying to do, it is sometimes so much instilled in them that some sub-optimal teachers will encourage students to master each step of the way before moving on, it is simply an inefficient approach.
I think there is nothing wrong with fooling around with pieces you are not ready for, in fact it is very important that you do extend yourself now and then but do not make this type of study your main focus. You should always do something out of your range, it is like the baseball player who swings multiple bats (or an ultra heavy bat) before going up to bat. In music there are many instances where we can swing these multiple bats before going back to our standard stuff, application in improving sight reading especially enjoys this type of study. When attempting pieces which demand technique from you that you have little or no experience with, try not to waste time perfecting these things, merely categorize and take not of what the technique is, notice what is most difficult for you and then move on, do not try to force it into improvement it will slow you down. You only labor hard and long with your difficulties in pieces you have been set and treat everything else as a long term goal and something to catalog.
With regards to the instruments, I really like larger grands as opposed to the smaller ones, however the Steinway is larger than a baby grand. I would NEVER buy a baby grand no matter what brand or quality, they are simply in my mind a waste of money because the length of the strings are no different to the upright, baby grand are merely to look pretty! Are you considering buying the instrument new? Personally I would NEVER buy a new Steinway, the prices are just too inflated. But it depends on the time you have, if you have time to search for a 2nd hand deal I would suggest doing it. I always search for good deals and I have offered a 9ft Steinway model D for little under 70grand as opposed to the 150+ k that you might pay for it new. Invest in a qualified piano technician to appraise all good pianos you are interested in, I have had technicians from other state look at pianos that I couldn't get to and purchased grands this way a few times never with any problems. Of course if money is no problems then don't waste your time, but for me at least there is much more satisfaction finding the right piano yourself rather than making some piano salesman very rich.
Hope you enjoy yourself here at pianostreet!