If you need to pad space with variable pitch font, and you are unable to
use ':align-to display' option for any reason, you can use the following
code. This uses unicode spacing characters
<https://jkorpela.fi/chars/spaces.html> to insert required amount of space.

*Screenshot:* https://lifeofpenguin.blogspot.com/2021/06/gnu-emacs.html

(defun company--insert-space (width w)
  "Insert unicode space characters to fit the WIDTH."
  (let ((num 0))
    (dolist (spc '((#x2001 .  1) (#x2000 . 0.5) (#x2004 . 0.33)
(#x2005 . 0.25) (#x2006 . 0.16) (#x200A . 0.125)))
      (setq num (floor (/ width w (cdr spc))))
      (if (memq (car spc) '(#x2001 #x2000 #x2004 #x2005))
          ;; Allow finer spaces towards the end for better accuracy
          (setq num (1- num)))
      (when (> num 0)
        (dotimes (i num)
          (insert (car spc)))
        (setq width (- width (* num w (cdr spc))))
        ))))