Ok thanks a lot for your answers, I made the function, if ever it can interest someone one day


(defun org-get-tags-with-hierarchy-at-point()
      "Return the list of tag WITH the sub-tags if they exist at point"
      (interactive)
      (let ((tags-heading (org-get-tags))
            (tags-result '()))
        (dolist (tag tags-heading)
          (dolist (tag-to-add (org-tags-expand tag t))
            (push tag-to-add tags-result)
            )
          )
        (delete-dups tags-result)
        )
      )

Do not hesitate to change the "tags-heading" variable to select a different heading

Thanks again for your answers !