What if the brain can interface with the device on a hierarchical chain instead of equally? It may be able to delegate computing tasks to the interface while retaining everything that is related with consciousness within the biological part.

You would have to see what is the minimum biological brain size where a person is alive and functioning well, then connect the interface and show that - with the device attached - you can reduce the biological brain to a size that would otherwise be impossible and still have them retain consciousness. Then it must be shared with the machine.