Avoiding one large Blackboard when using Behavior Trees with Subtrees

For the AI of my NPC I have several (small) Behavior Trees for various behaviors (e.g. FLEE, PATROL, HIDE). There is one master BT which calls those behavior subtrees based on the information in the blackboard. Each of the subtrees have their own configuration options in the blackbaord, so I end up with one large blackboad used in the master BT, which in turn includes all the configuration options for the subtrees.

Now this large blackboard has become quite confusing with dozens of different keys for the various subtrees. However, I can use only one blackboard for the master BT, thus creating smaller blackboards for each subtree does not work, as I don’t call these subtrees directly.

What is the best way to handle blackboard information/keys for such large behavior trees with multiple subtrees?