There might be a simpler way as follows.
Create $AllTags and load it with the set of the 50 canonical tags (by setting it’s “default” in the user attribute inspector)
Create $MissingTags as the set of tags that are missing in the current note
Assume you are using the standard $Tags attribute for the tags in the current note
Create an edict or a stamp or an agent with this action
$MissingTags=$AllTags - $Tags
and you will find in each Note’s $MissingTags attribute the set of tags that that note does not have.