OK, I'm new here and am not yet any sort of pianist, but have been around the block a bit with other instruments so take this with a pinch of seasoning. To start with, I think your question is good and I'd also like to hear similar stories (being a very late starter with significantly lesser ambitions than yours).
I would feel really hollow if I found out that it's unlikely I will ever be able to play them well because I've passed a critical window of learning opportunity or some other bs reason, and I might as well rather just give up trying to play the piano.
Suppose you met an oracle who said it could tell you whether you will ever be able to play the Hungarian Rhapsodies. Would you want to know? What would you do if you got the answer "no"? I mean, what use would that information be to you?
So what if nobody has ever started piano at 20 and mastered the Hungarian Rhapsodies? You can be the first. Someone may as well be, assuming it isn't
impossible to achieve that. I don't mean that in a pep-talk way, just factually. For every achievement, huge or trivial, somebody did it first and up til then nobody had done it before.
Adults and children clearly learn things (all things) in different ways, and have different advantages and challenges. I don't know much about piano teaching or learning, but I can't believe that with a dedicated practice regimen and good training you won't be able to reach that standard eventually.
Another way to look at the oracle's answer: It seems to me this is a bit like knowing the day you'll die; isn't it better to proceed hopefully and see how far you can get?
Behind this is one of the Great Truths that they don't tell kids about: you never really "get there" in any area of life. You never "get good", you get better and your goals change as you do. The goal is always receding from sight and that's just how things are supposed to be. I suspect most concert pianists don't sit around gloating about how good they are; they practice and fret about how far they are from their own goals, which perhaps you and I can barely imagine.
I think of being successful as being able to produce on the piano anything that I can imagine.
This is a ridiculous thing to expect to achieve. Or it's not, depending on
what you can imagine. There are musicians (I've met some) who can play anything they can think of because all they can think of are the same few simple things they've played all their lives. But if your ears and mind are open, the thing is that the more music you learn and absorb, the more you can imagine. Be proud that you can still imagine what you can't yet play; be worried if that changes.
For what it's worth, I would be delighted if I could play at the level you describe yourself as being at now. You're
my goal, Hungarian Rhapsodies Person is
your goal, and so it goes.
Sorry none of this answers your question which, I repeat, I consider to be a good one
