iTunes won't open at all but it appears in task manager

Try the following in turn until iTunes opens normally:

• Locate the shortcut for iTunes, right-click on it, and select Run as administrator. Doing this once may fix certain launch issues. Other users are finding that with the 12.6.1.25 build they have to use this method every time.
• Try holding ctrl+shift as you launch iTunes so it opens in safe-mode. Again doing this once may sometimes help.
• Delete the iTunes shortcuts from the start menu, desktop, task bar, or similar, then repair iTunes from the programs and features control panel.
• Try Fix unexpected quits or launch issues in iTunes for Windows - Apple Support and test the behaviour in a different user profile or with a different library. If the problem only exists in your profile it may be possible delete some of the iTunes preference files which it would seem that iTunes may be choking on. We can go into details later if the issue is limited to your own profile.
• See the second box in Troubleshooting issues with iTunes for Windows updates for detailed tear down and rebuild advice.
• Or see the iTunes installers section and reinstall the last version that worked correctly for you.

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The general troubleshooting advice when iTunes fails to install properly (and reinstalling or repairing it does not work) is to remove not only iTunes but all related components, reboot, remove the target installation folders and reinstall. See the Removing and reinstalling iTunes and other software components section of turingtest2's article at:

Troubleshooting issues with iTunes for Windows updates

There is the possibility that the underlying cause is some form of permissions issue which running as administrator bypasses. See Repair security permissions for iTunes for Windows. You could try a similar approach on the Apple related folders inside the various branches of your <User>\AppData folder.

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Let Apple know via Feedback - iTunes - Apple or make a free registration for the Apple Developer Program and then make use of Apple Bug Reporter to report the issue, and use the workaround in the meantime.

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Windows Defender shouldn't cause any AV related problems with iTunes.

Opening iTunes in its safe mode makes some other changes in addition to disabling plugins. You probably don't have any.

You can edit the properties of the iTunes launch icon to run as administrator which might work.

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Did you try 12.5.5.5? This seems to be relatively stable as far as I can tell.

Did you disable any anti-virus and run the installer as administrator?

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The only way it opens is through safe mode (ctrl+shift).

Do you still have any 3rd party plugins or scripts that you have not removed?

Launching completely in safe mode proves it's a 3rd party issue.