SLS is expending surplus RS-25 shuttle engines. Once they're gone they'll be paying Aerojet Rocketdyne a hundred million each for new ones. The first stage uses four.
Eventually someone with a brain is going to notice, SLS will be canceled, and decades of federal jail time will ensue for the criminals who perpetuated this fraud upon the American people.
ISS involved assembling segments that could be lifted on the Shuttle. Continuing that would involve lifting segments that could be lifted on F9 or FH. There would be no need for something as large as SLS.
SLS will be delivering segments of Gateway after the first 2 are delivered by Falcon Heavy.