I want to understand how you are planning to implement Canonical Tag using DTM? Are you going to write something in DTM client script and injecting canonical tags like other tags? You can't add logger in below client side script or something to check if canonical tag is executing or not?
Dynamic Tag Management - Digital Marketing cloud service that allows a marketer to manage Adobe and third-party tags used for tracking or other analytic purposes. It is done through client-side scripting that injects tag related code throughout the pages of the site.
I am not aware of DTM, so not sure if you can inject other tags from DTM or you need to implement something in AEM.