You can postpone the update for some time ,You can unselect Windows Defender in Sledgehammer.cmd,
but Windows Update shall update it after some time.
You can delete it and use another virusscanner/protector. , there are many of them., see onto the internet
I use Malwarebytes for years without problems.
Concerning security center, it will survey and it will tell You your system is unprotected when Your Antivirus is
diabled , be it Defender or any other antivirus that is installed
Sorry no working solution for Windows Defender.