This is a weird way around your request, but hopefully it helps.
If you drop an empty file called running.lock in this directory, before starting the sim, MSFS will prompt you to use safe mode. This is for MS Store version, not sure what it would be for Steam.
C:\Users\%username%\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\running.lock