Elastic Email, one of your recommended email services, will always add an unsubscribe link at the end of any mail sent through their service in order to comply with anti spam legislation. There is no way that this can be disabled, not even by contacting support.
It is clear that managing unsubscribes in a separate application (i.e. Elastic Email) is not really a great option. I think what @mmiada meant, and I would like to ask as well, is the following:
Does Discourse provide any way to disable all emails for a given user by making a http request to Discourse from an external application, with the email address of the unsubscribing user as a parameter? This would allow any unsubscribes coming from Elastic Email to disable mailing for the unsubscribing user, because Elastic Email offers this capability in their system:
The Elastic Email http notification specification is documented here.