In the Laravel documentation, there is the following example for retrieving morphedByMany relations, which are many-to-many polymorphic relations.
morphedByMany
Larav
You should be able to add a relationship on your Tag class as such
public function taggable() { return $this->morphedTo(); }
That will use the taggable_id and taggable_type to get the relevant Model that it is tied to.