Win 10 upgraded to Win11 screwup (opens in new tab)

I recently upgraded my computer from Windows 10 to Windows 11, and something has gone seriously wrong. The upgrade process seemed to complete without any error messages, but now my system is a complete mess: my taskbar icons are missing or unclickable, the Start menu won't open at all, many of my apps crash immediately when I try to launch them, and the overall performance is painfully slow even though my PC ran perfectly fine on Windows 10. I've already tried running the Windows Update troub...