The 7th Sonata is a monument of a work, and make no mistake, very difficult.
And what is so difficult, IMO, is bringing out the "back story" theme of common man trying to survive in a dystopian society, which is what this is all about.
If you can find it, listen to the Horowitz recording of the whole thing, which IMO gives a very good "character study" of what the piece is about. Also there is Richter's version, also great.
Most pianists make the mistake of playing the last movement too fast, which destroys the point of it. Listen in the link to Richter playing the last movement, bringing out the relentless, crushing savagery of the dystopian machine. Played too fast, and the rhythm is destroyed, and the effect is gone.
Richter was the one who premiered the work having learned in 4 days.
https://www.newmillenniumrecords.com/classroom/room42.htmlI think the 1st movement is the most difficult technically, and the 2nd musically.
The last movement's real difficulties lie in the last two pages. Again, if you lose your nerve and speed up throughout the last movement, by the time you hit the last two pages, you are DOOMED!
It is NOT the speed of the last movement that is impressive, it is the controlled savagery of the thing, trying to keep it in its box!