HISM is just a component you can add to blueprint, and use that to actually spawn the tree instances. So then you could just update the HISM instances with whatever the tree manager is doing. I’d expect HISM instances to be much more performant than individual actors.
It’s really not as complicated as it sounds. Simply add Hierarchical Instanced Static Mesh component to your tree manager class, and you will see it has functions like Add Instance, Remove Instance, Update Instance Transform, and so on 