Hello, and welcome to the Forum.
Your hedges appears to be composed of Shrubby Yew-pine (Podocarpus macrophyllus var. maki), an evergreen conifer native to Burma, China, Taiwan, and Japan.
It is good taht you are considering watering amounts and drainage. Af ter establishment it may need little suplemental irrigation in your climate.If you feel that watering and drainage are correct, I would just continue further observation at this time. Leaf drop is only a problem if over the course of a year more leaves fall than are replaced. All woody plants eventually drop their oldest leaves, the ones farthest from the growing tips. This is the pattern we see here. Many evergreen conifers do drop their oldest leaves in the Autumn. If the plants continues to fill out overall, I would not worry bbout the level of leaf drop I am seeing.
The fertilizer spikes sound like an fine addition, even if they don't stop the leaf drop.