By definition, GSX/Couatl CANNOT CRASH THE SIM, since it runs completely external to the simulator, under a separate .EXE ( the Couatl engine ), and by definition, an regular .EXE like that doesn't have ANY access to the simulator process or memory, this is enforced at the Windows level.
A corrupted EXE.XML instead, CAN possibly crash the sim at start, and the message you saw during the installation is making it clear your already corrupted EXE.XML wasn't touched by GSX, so you can be sure the problem wasn't caused by the GSX installer, which won't dare to touch an already corrupted EXE.XML.
I suggest removing the EXE.XML file altogether, then reinstall GSX again, which will then create a new one which is surely not corrupted. The file can be found here:
MS Store version
%LOCALAPPDATA%\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\exe.xml
Steam version
%APPDATA%\Microsoft FlightSimulator\exe.xml