You can use the assembly restructuring functionality to break down large routing assemblies into manageable sections. The components must be fully resolved (not lightweight) to perform these operations.
You can edit the structure of a routing sub-assembly without losing any in-context features that it contains. However, you can move routing components involved in in-context features only within the sub-assembly (and its nested sub-assemblies) where the reference was created.
You can form a sub-assembly from selected components in a routing sub-assembly. With the top-level assembly active, select one or more components (hold down Ctrl when selecting multiple components). Then right-click one of the components and select Form New Sub-assembly Here. The new sub-assembly does not contain a route of its own; it is driven by the route of its parent assembly.
You can create nested sub-assemblies, with as many levels as you want.
You can move a component of a routing sub-assembly to a parent or child assembly that uses the same 3D sketch to drive the route. You can either drag and drop the components, or use Tools, Reorganize Components. You cannot move a routing component outside of its own branch of the hierarchy. You cannot move components from outside the routing sub-assembly into the routing sub-assembly.
You can dissolve a routing sub-assembly if its parent is a routing sub-assembly. Right-click the nested sub-assembly, and select Dissolve Sub-assembly. You cannot dissolve a routing sub-assembly that contains a route and its 3D sketch; therefore, you cannot dissolve the routing sub-assembly that is the top-level assembly component.