OK if it were me, I’d do a Clean Install of my Avast Installation and then make sure Windows Defender is Disabled. The installation should disable it but I’d make sure.
Reboot again.

“Clean Install”
Down load ‘and save to your desktop’ the Avast Uninstall Utility here: https://www.avast.com/en-gb/uninstall-utility

Download to your Desktop your correct Avast version from here: https://forum.avast.com/index.php?topic=178580.0

Now disconnect from the Internet
Uninstall Avast via the Control Panel

1.Run the uninstall tool and accept the reboot to safe mode
2.Once complete reboot your system
3.Reinstall Avast