For those that are having issues with MSFS starting.
If you limit the affinity to anything but all cores on MSFS, it will crash on loading. Instead of trying to limit the cores the simulator uses, limit all background programs to anything but your main threads that the sim is using.
So, with process lasso open, and the sim running. Monitor which cores get the most activity when the sim is running. Then in process lasso make sure that all background applications and programs are set to affinity mask with out the one the main thread is clearly on.
So, in my case with my 7800X3D the simulator uses cores 12-13. With that I set the affinity mask for all background programs to be 0-11. This ensures that the background applications don’t cause overloading of the main thread on the simulator.