Thank you Paolo for the interesting considerations, and excuse me but I only saw your response today. I remember that you have often mentioned Cortot's book but I don't know it, so I will see to finding it; the classification of the most famous piano compositions according to difficulty is very interesting; indeed, as a almost total self-taught player, I often choose new pieces to learn based only on the impact of listening to the melody and harmony, without taking too much account of the level of difficulty, so that some turn out much easier, others much more difficult.
In a certain sense, I understand what you mean about the fact that studies, however, lack things that are only found in larger-scale structures; I imagine you are referring especially to the 'development' sections which, in the context of more extended pieces, require more 'remote' contrasts, more distant modulations, etc.
In reality, I am starting to practice pieces that are not studies as well; for me, studies are a bit like a 'magic potion' to go beyond the beginner-intermediate level, in short.
Curiously..... I have been living in England for quite a while and here there are fairly precise systems to 'verify' a pianist's level, for example the ABRSM system; you start from Grade 1 up to Grade 8, the maximum (then after, I think comes the concert pianist level, I am not sure. Unfortunately, I don't know how it works in Italy).
Well, two years ago I decided to take lessons at a school, for the first time because, in effect, by doing it alone I think I have done everything I could. Incredibly, at the audition, while playing half of Mozart's Rondo alla Turca, they passed me to Grade 6. I couldn't believe it.
This piece was the first piece I began to learn (I did everything in reverse), but I didn't even know how to play the final part. I think they were impressed by the fact that I played the first half of the piece quite well.
To say..... for me Grade 6 is completely undeserved, I feel like a 4, yes or no. I think I will have to write a new topic about it. Thank you Paolo for your good advice and thank you anyway to everyone. Long live music