Maybe the mining of the planet could be used to imply that the planet’s the source of all the Star Destroyers also.

So is that why you’re leaving the hollowed out parts? 'Cause personally I would just make it an unaltered Ilum, no mass mining involved.

Perhaps the scene where the Hosnian system is destroyed could be recontectualized to be an attack from a First Order Dreadnaught.

I’ve been saying this literally since before Dreadnoughts were even a thing. The fact that they are makes it all the more approachable. The fact is the destruction of the Senate is important. What isn’t is the destruction of an entire star system.