It seems like you have already taken the correct steps to disable the notifications. However, there are a few things you could check:
Group Policy Update: Ensure that the Group Policy has been updated on the client machine. You can force an update by running the gpupdate /force command in the command prompt.
Registry Check: Check the registry settings on the client machine to ensure that the Group Policy settings have been applied correctly. The settings should be reflected in the registry keys you mentioned.
User Permissions: Ensure that the user account has the necessary permissions to apply these Group Policy settings.
Conflict Resolution: There might be a conflict with another policy or local setting that is overriding your Group Policy setting.
Software Updates: Make sure that Windows and Microsoft Defender are up-to-date.