A few weeks ago I repotted a dawn redwood forest that has been in the same kind of Coco mix for two years after purshase. It was a similar situation. Lots of hollows in the soil/rootball. However not the optimal for developing a nice root structure it did not pose any harm to the health of the trees.
I stand firm in my belief that beginners are best to grow trees a year or two. Learning the species and its care before repotting.
I would recoment to add a drainage screen to the pot and fill the hollows with a non organic substrate.