You've tried some fixes from blogs, but I'll give you my list of diagnostics in the hope that it might help you. You may well have done all or most of them already.
Deleting Preferences
Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
Opening iMovie in a new library
Try opening iMovie in a new library. To do that, open iMovie while holding down the Option key and then selecting to create a new library in the box that appears.
Opening iMovie in a new user account
Create a new user account from System Preferences/Users and Groups and try to open and use iMovie in the new account. You can use the Guest account on your Mac for this purpose.
Safe Mode
You could also try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly.
Updating iMovie
(You've already tried this one.)
Resetting the System Management Controller
Try resetting the System Management Controller (but no longer available for the new M1 computers):
How to reset the SMC of your Mac - Apple Support
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM, although this issue would be fairly rare:
Reset NVRAM or PRAM on your Mac - Apple Support
Hopefully one of the above will isolate the problem.
Good luck with this. If you talk to Apple Support, and have time, it might help others if you could post back what they said.
-- Rich
