Currently I just serialize and write a TArray with the FTransform of each instance to a .save file, which I can load in and feed to my mesh component by using AddInstances(...) .
Does anyone else know a better/faster way to save&load Hierarchical Instanced Static Mesh Component?
In my opinion better way is to use Instanced Foliage Actor.
Iterate through all of your Hierarchical Instanced Static Mesh Components, for each create Instanced Foliage Actor, set static mesh, then loop from instances count and add each transform.