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>Is it possible to hide the ‘Windows preparing’ screen
I am afraid not, I can’t find and never heard a way to make it.
But, for your situation, I find out an really old Microsoft article, even though it is aimed at Windows Vista we can still use this registry value.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System
DelayedDesktopSwitchTimeout=0
Besides, some users here talking about similar phenomenon, maybe can give you a hint.
Displaying a message during a powershell login script on Windows 10
https://www.reddit.com/r/sysadmin/comments/7eqerr/displaying_a\_message_during_a\_powershell_login/