QuickTime crashing when trimming

If safe mode did not help, try this:

QuickTime Player > Services Settings... > Restore Defaults.

Or maybe:

Settings > Desktop and Dock > Click wallpaper to reveal desktop: "Only in stage manager"

Settings > Desktop and Dock > Stage Manager: OFF

Or make a dummy user and see if Quicktime Player works in that account. If that works, try to reset QuickTime preferences in your account: Quit Quicktime Player, and try if moving these folders to the Desktop folder (just in case before deleting them) helps:

~/Library/Containers/QuickTime Player

~/Library/Containers/com.apple.QuickTimePlayerX/Data

If that does not help, use some 3rd party app like Avidemux to trim the movie.

Did you try restarting? Try restarting in Safe Mode. Does the same problem occur while in Safe Mode?