I know there’s been issues as well with the version of windows for some people. Windows 10 vs 11 and also the last service pack update from windows caused issues. Perhaps rolling back to a previous version if you’ve recently updated?
Also what type of HDD are you installing it on, and have you tried installing it onto a different drive?
The large file I was referring to was the one the admin commented C:\Program Files (x86)\Steam\steamapps\common\MicrosoftFlightSimulator
C:\Users\YourUsername\AppData\Roaming\Microsoft Flight Simulator\Packages
that’s the default location unless of course that was changed by yourself during the initial install.