Dear all. I get that it may be a tad complicated, but if anyone sees at least a reference about how it may be done, please do share it here. Thanks a lot, in advance... :)

Edit 2026-01-07: added the missing ) and fixed the formatting

Here's how I would approach it:

#+STARTUP: noptag
#+TAGS:
#+TAGS: [ GT1 : tagA tagC tagD ]
#+TAGS: [ GT2 : tagB tagE ]
#+TAGS: [ GT3 : tagB tagC tagD ]

#+NAME: source
| tag  | Q1 | Q2 |
|------+----+----|
| tagA |  9 |    |
| tagB |  4 |  2 |
| tagC |  1 |  4 |
| tagD |    |  5 |
| tagE |    |  6 |

#+begin_src emacs-lisp
(defun my-sum-tag-groups (source)
  (cons
   (car source)
   (mapcar
    (lambda (tag-group)
      (let ((tags (org--tags-expand-group (list (car tag-group)) org-tag-groups-alist nil)))
        (cons
         (car tag-group)
         (seq-map-indexed
          (lambda (colname i)
            (apply
             '+
             (mapcar
              (lambda (tag)
                (let ((val (or (elt (assoc-default tag source) i) "0")))
                  (if (stringp val)
                      (string-to-number val)
                    (or val 0))))
              tags)))
          (cdr (car source))))))
    org-tag-groups-alist)))
#+end_src

#+begin_src emacs-lisp :var source=source :colnames no :results table
(my-sum-tag-groups source)
#+end_src

#+RESULTS:
:results:
| tag | Q1 | Q2 |
| GT1 | 10 |  9 |
| GT2 |  4 |  8 |
| GT3 |  5 | 11 |
:end: