I've played piano over 40 years. By the time I was 25 I played most of the piano repertoire, all 32 Beethoven sonatas etc. That said, let me offer my since advice on how to climb to the top of piano performance. Anyone can do it and this method works but you won't hear about it much and I'm unaware of anybody promoting this method. OK.. Here goes.... I love playing easy pieces. There is so much excellent low laying fruit which everyone should pick. Here are some examples... 1. Bartok - Mikrokosmos (all 153 pieces - play them all, you can do it!) 2. Prokofiev pieces for children. 3. Schumann 43 pieces for children. 4. Bartok 20 pieces for children. 5. Bach 15 inventions and 15 sinfonias. 6. Bach little fugues. 7. The easiest Mozart sonatas e.g. K282 etc. 8. The first 10 Haydn sonatas (partitas). 9. Well tempered clavier - Bach. etc. 10. Select pieces from the French and English suites. ... well you get it.

The whole point is to start small, learn pieces quickly, but most importantly musically. Make music with them. Learn to read from the score comfortably. After a while reading things at sight isn't such a big deal.

The important thing is to learn music is a way that solid skills are acquired. You certainly can focus work on a Beethoven sonata for several months like at a conservatory but I believe this method of learning vast amounts of music quickly is absolutely indispensable. There are a TON of excellent first rate compositions by the best composers, you have no excuse for not mastering them.

The gap between being able to play Haydn sonatas and the Well Tempered Clavier fluently and say being able to work on the "big pieces" is much smaller than you think.

Don't skip anything. Nobody needs to charge in and start playing Beethoven's sonata op. 57 The Appassionata before they've played at least 20 of the easier sonatas.

The other bit of advice I have is to constantly work your memory. Again with simple pieces learned quickly. Most pieces don't need to be memorized and memorizing everything will hamper your development. You do however need to memorize easily and efficiently. It really helps in developing speed and accuracy. Chopin studies mostly need to be memorized in order to practice effectively. That said, I firmly believe you can learn MORE technique playing finales to Haydn sonatas but with a whole lot of energy and spark than slaving away on virtuoso etudes.

One last thing, practicing without pedal (often but definitely not always) is very helpful. It has been for me.

Oops. One more last thing. Play by ear and improvise in any style you like. You'd be surprised how many super advanced classical musicians there are who can't play Happy Birthday at a moments notice without knowing the song at all just to make a group of children happy. This is a symptom of the lop sided skills not to be able to play songs people like whether it's Carol King or Arcade Fire. You should be able to fake something.

TLDR; Picking all the low laying fruit you can find is the one way to success. You get to focus on enjoying the process of actually making music than to struggle with passage work that always comes on its own anyway.

Good luck.