I’m not seeing anyway to only track successful clicks on the signup button. You could try tracking clicks of the “Click here to activate your account” button:
It is also possible to track signups with a User Event webhook. That is a very different approach than what you are currently using though. You’d need to configure an external service to listen for the webhook and then filter the user webhooks to only collect data when the X-Discourse-Event: user_created header is set in the request.