Sorry but you can’t differentiate users like that.
Zammad will always create a new customer account if the user that drops a support request is yet unknown to Zammad. This is a core function and required for Zammad to function.
So you can go for option one and send the same trigger upon any ticket creation.
Zammad provides a default trigger that by default is enabled that does the same. May need adjustment through.
maybe this here is an approach to differentiate between users that are already known in ZAMMAD and “new users” in a different way:
Because Zammad registers all new Users and creates an account for them (which normally is pretty cool) you might identify them because they are not assigned to an organization (if you do not use “Domain-based assignment” for the organizations).
So that means for turning “unknown users” into “known users” you have to bring a process in place (e.g. manually) to add all known user to an organization when they should be treated as “known users”, while the new users are not assigned to an organization.
Then you can use a condition in triggers “Customer-Organization” “is not” “not set (not defined)” to identify the existing, validated users to send a different mail to them.
In the trigger for the new, unknown users you then use the condition “Customer-Organization” “is” “not set (not defined)” and send them a different mail (until you assign them to an organization and they become a “known user”.