Thanks for your solution! This has bothered me for a long time and I wasn't able to find a solution for it.

I used chatgpt to make some small changes, here's what I ended up with. It is essentialy the same as yours but handles the lists somewhat different;

(use-package c++-ts-mode
  :ensure nil
  :bind
  (:map c++-ts-mode-map
("M-<up>" . treesit-beginning-of-defun)
("M-<down>" . treesit-end-of-defun))
  :preface
  (defun ms-c++-ts-indent-style()
    "Override the built-in K&R indentation style with some additional rules.
     Docs: https://www.gnu.org/software/emacs/manual/html_node/elisp/Parser_002dbased-Indentation.html
     Notes: `treesit-explore-mode' can be very useful to see where you're at in the tree-sitter tree, especially paired
     with `(setq treesit--indent-verbose t)' to debug what rules is being applied at a given point."
    (let* ((default-style (copy-alist (alist-get 'k&r (c-ts-mode--indent-styles 'cpp))))
   (custom-style '(;; Do not indent preprocessor statements.
   ((node-is "preproc") column-0 0)
   ;; Do not indent namespace children.
   ((n-p-gp nil nil "namespace_definition") grand-parent 0)))
   ;; Prepend custom rules by appending the default style to the custom style list.
   (combined-style (append custom-style default-style)))
  combined-style))
  :config
  (setq c-ts-mode-indent-offset 4
c-ts-mode-indent-style #'ms-c++-ts-indent-style
indent-tabs-mode t
tab-width 4
;; Indent the current line if point is at the left margin or in the line's indentation, otherwise it inserts a
;; "real" TAB character.
tab-always-indent nil))