Schubert's late compositions cannot be compared to anything he wrote during his lifetime.
If you want a stepping stone to the impromptus, try the moment musicals. His last three sonatas, in my opinion, are some of the most difficult examples of the form and take an immense amount of musical knowledge, as with many composer's "end of lifetime" works, to play well. Not to assume that you aren't capable of playing the second movement well, but it seems you'd rather focus on an impromptu - none of the movements of his last sonatas are stepping stones.
I've always found this movement to be a mourning of Beethoven than anything else. It's a remarkable piece of music.
Best wishes,