I have been learning D537 (A minor) I would say that this piece is around Grade 8 standard. But - I have to say I prefer the late piano sonatas (to hear as I haven't
tried to learn them.)
What I like so much about Schubert is the peace and melodic nature of his piano music.
I'm learning Beethoven's Op90 piano sonata at the moment - I think its second movement is very Schubert like - that's worth taking a look at.
I was given the following list by a Schubert expert (a concert pianist) - as places to start :-
D664 (slightly harder than D537)
D157,D557,D459 - (early and incomplete sonatas)
Best of luck