TBF ISIS’s belief structure always seemed more heavily influenced by America (especially right wing US Christian fundamentalists) than it was a standard, back to real Islam movement of the kind that seems to emerge every second generation - as in that it had to be spectacular, the world had to shake and achievements presented to the masses rather than prizing modesty and simplicity of belief.
They wanted people to go over there and presented the idea of riches, slaves, and glorious victory. They were finally going to rid the world of traitors, heretics etc unlike every other regime in history, their country was going to be the the best and so on. I am not sure it was ever going to be that much about Sharia as it would have been about supremacy.