I think what's going to actually happen is some version of this, with a couple minor modifications.
I don't think there's any chance they simply waive Simons, essentially 0%. They aren't going to want 9 million a year for three years stuck on their books. I do think they will try to trade him, not necessarily a pure dump but for somebody making less to save them some tax. One example would be Simons for Vucevic, saves 6 million dollars and gives the Celtics a starting quality center for next year. I do think they will for sure dump Niang at some point, but maybe not until the deadline.
I hope that Horford is back. Feels like its golden state or retirement, but if that's the case not sure why he's taking so long to decide.
If their goal is to get below the tax, well they have a lot of work to do. They are about 21 million over. Dumping Niang would save 6, dumping Hauser (which I don't think they want to do) saves 8. They HAVE TO trade Simons for less money coming in to have any shot at it. I could very easily see them simply deciding to stay 5-15 million over if getting under becomes too damaging to future rosters.