Great story!
I would like to see a blend of options 2 and 3: Keep the domination good natured and loving, but let there be some hints of aggression without being overly mean. I like seeing the potential for darkness, even if it never fully manifests. It can make a sub even more grateful for "good natured" domination, knowing where it could go if things went badly. Alternatively, the sub could develop a curiosity for what hides in the darkness.
In any event, great start and I hope there will be more to come.
K+