I have two Schnittke recollections - the first is listening to the Proms in the 80s on R3 and hearing the Beethoven Violin Concerto like Id never heard it before- with the Schnittke cadenza. I was outraged I remember- that someone would have the temerity. The next time was in 1998 at a dinner party held by a psychiatrist who had studied composition at the Royal College of Music in Dublin. He decided to entertain the guests on the piano and I foolishly (jokingly) asked if he knew any Schnittke. He said “do you like Schnittke?” His wife I noticed had made a tactical exit from the room. Within minutes, guests were running for the door. I have a mental picture of Edvard Munch’s the Scream. We are still friends 20 years on.
To come back to the OP’s question “Schnittke- where to start?”- my answer would be, “Don’t”.