This page might help you..... https://japanobjects.com/features/sumie
I don't know much about Sumi-e painting but obviously practice helps, as with all painting, so the more you practice the simple brush stroke, loading, placement, pressure etc, the better you'll get!
It seems to me to be very direct.... the mark from the brush, rather than a shape painted and filled, is the essence of the art ... then the combination of the marks, the composition of various elements, as you've done here, make the story.
I think essentially, sumi-e is not laboured but a carefully planned work before putting ink on brush!
That's just my reading of a painting!
I think if you work out how to hold the loaded brush, carefully place it on the paper where you want it to go and carefully apply pressure, you'll get a beautiful shape of the leaf.
For stems, maybe a loaded brush drawn smoothly up the paper in the desired direction will give you the dark side, then perhaps repeat with a water-loaded brush up one edge of that line will draw the ink out to make the lighter side.
Will you try these ideas and tell me what you think?