How to slim down assembly?

Good day fellow 3D wizards,

After 5 years my manager decided that there are too many parts in a single assembly and wants me to slim down the main assembly into 10 - 15 sub assemblies.

Is there an easy way to do this? Without having to build complete new sub assemblies or shooting down an 5 year old model.

I have been trying to add an assembly and dragging parts into that but that messes up all the mates, patterns and mirrored parts.

SolidworksAssemblies