Summarize the theme of Siddhartha as more elaborate than "a journey into the depths of the human soul."
I think that much of this is going to depend on how intensely descriptive you wish to be. You are right in that the "journey to the depths of the soul" element is a bit cliched in discussing Hesse's work. Part of the reason why it's cliche is because it's safe. I mean, it's not really wrong. Your comfort with the work is going to represent how intricate you would like to go with this. For example, stressing the overall transcendental tone of unity and interconnection could be one way to go. Siddhartha ceases to struggle when he recognizes that there is an interdependence of everything and that he is a part of this. When he loses the sense of "I" or of the strict subjective and starts to examine life and being in the world as part of a larger element, a manifestation of something larger, peace becomes more evident. Instead of focusing on binary dualism or arbitrary labels that divide, Siddhartha demonstrates peace when he understand the totality of being in the world, the life force that permeates everything can be recognized in everything. This allows for a greater sense of harmony and unity as it recognizes the truly infinite nature of reality. The discussion of the stone might demonstrate this:
This stone is stone; it is also animal, God, Buddha. I do not respect and love it because it was one thing and will become something else, but because it has already long been everything and always is everything. I love it just because it is a stone, because today and now it appears to me a stone
The totality of experience is in everything and once Siddhartha recognizes that life is "indestructible" and not something that is finite, but rather that the experience of life is transcendental and in everything, a peace and calm settles inside him as he receives a glimpse of being in the world.