I can't delete the default Google Oauth authentication from Lacework, it's greyed out when I try.
This is by design. You cannot delete the original Google Oauth method to ensure that there is always a way back into your platform should you remove another authentication method.
You can only have one authentication method active at a time so having another authentication method automatically disables the default Google Oauth.
Find out more here: https://docs.lacework.com/google-oauth-configuration