> 'split-window-preferred-function' is well suited for a completely different
> behavior.

There is another aspect that still can't be customized.
The choice of the window to split is hard-coded
in 'display-buffer-pop-up-window'.  It's either
the largest window or lru:

  ;; Attempt to split largest or least recently used window.
  (setq window (or (window--try-to-split-window
                    (get-largest-window frame t) alist)
                   (window--try-to-split-window
                    (get-lru-window frame t) alist)))

Maybe another option needed here as well?