As exciting a rocket the SLS is, really, it is two expensive, too late. And by expensive, I don't mean "throw taxpayer dollars at it" expensive. I mean "each of the four engines costs $160,000,000 so we're going to empty the museums of RS-25's to save money" expensive.
Understand that just one engine - out of four - on the SLS costs more than a Falcon Heavy flight. Starship will cost an order of magnitude less per flight, and are aiming for _two_ orders of magnitude less.
To be clear: one engine alone for SLS costs ten times more than an entire Starship flight. Both are paper rockets for now, so the comparison is apt. And the SLS has four of those engines, plus everything else: the H2/O2 core, the SRBs, and the Orion capsule, to add to the cost.