Both of your approach should work, but I would go with the lifecycle state option (This depends on your business use cases as well) because of the easiness to trigger the enable/disable operation during the lifecycle state changes. This would help you even if the user moves back to an active status from a terminated status by enabling their accounts as well as assigning the required access profiles.
Also, the recently introduced Identity states as described here can also help you with your Identity processing time or exclude them from the Request center.
Either way, you will need to setup a before provisioning rule or use the Services standard before provisioning rule to achieve the account deletion through ISC. Else you will need to depend on an after modify powerShell script for AD account deletions.