Morning sun and partial sun in the afternoon is just fine. Bring the mulch back away from the base of the trees. Mulch should be like a donut, not like a volcano. Fertilize in early spring with a slow-release fertilizer. The yellow leaves are probably caused by not enough water. Birch trees need water. And, watering trees is different than watering the grass. Trees need deep watering every week or so, especially when getting established. If getting out and watering is difficult, try a tree watering bag. They do a great job.
Thanks for writing!