Currently webhooks are only supported for new survey records (adding new data), not editing existing survey records. We currently have an open enhancement request for this feature and I have updated the issue with your comments/request.
Webhooks are great, however the current implementation is useless for our workflow. We use the Survey123 Inbox feature with a query for scheduled inspections. It would be great if a webhook could be triggered on the Send Now event when we complete a survey from our inbox.