why on earth would you take your own planet’s core to build a death star? You’d pillage somebody elses. Plus obviously you’d enslave some people to build it for you.
Imagine how many delivery ships you’d need every day to supply a death star….. It would be a constant stream of ships with food, water, condoms, those little things you rest your wrists on when using a computer, pencils, leather chaps… the list goes on and on.
And you’d have to leach every planet you came near to achieve this. You certainly wouldn’t go around blowing them up, they’re generally full of resources and ladies. Blowing them up before stripping them bare makes no sense.