Seems to be working fine. Tested on Windows and macOS (with global menu).

There are few extra lines that can be removed (called if menu style is changed in runtime):

diff --git a/editor/editor_node.cpp b/editor/editor_node.cpp
index cc070096c2..dc918500fc 100644
--- a/editor/editor_node.cpp
+++ b/editor/editor_node.cpp
@@ -7778,13 +7778,6 @@ void EditorNode::_update_main_menu_type() {
                }
                memdelete_notnull(main_menu_bar);
                main_menu_bar = nullptr;
-
-               if (project_run_bar != nullptr) {
-                       // Adjust spacers to center 2D / 3D / Script buttons.
-                       int max_w = MAX(project_run_bar->get_minimum_size().x + right_menu_hb->get_minimum_size().x, main_menu_button->get_minimum_size().x);
-                       left_spacer->set_custom_minimum_size(Size2(MAX(0, max_w - main_menu_button->get_minimum_size().x), 0));
-                       right_spacer->set_custom_minimum_size(Size2(MAX(0, max_w - project_run_bar->get_minimum_size().x - right_menu_hb->get_minimum_size().x), 0));
-               }
        } else {
                main_menu_bar = memnew(MenuBar);
                main_menu_bar->set_mouse_filter(Control::MOUSE_FILTER_STOP);
@@ -7815,13 +7808,6 @@ void EditorNode::_update_main_menu_type() {
                memdelete_notnull(main_menu_button);
                menu_btn_spacer = nullptr;
                main_menu_button = nullptr;
-
-               if (project_run_bar != nullptr) {
-                       // Adjust spacers to center 2D / 3D / Script buttons.
-                       int max_w = MAX(project_run_bar->get_minimum_size().x + right_menu_hb->get_minimum_size().x, main_menu_bar->get_minimum_size().x);
-                       left_spacer->set_custom_minimum_size(Size2(MAX(0, max_w - main_menu_bar->get_minimum_size().x), 0));
-                       right_spacer->set_custom_minimum_size(Size2(MAX(0, max_w - project_run_bar->get_minimum_size().x - right_menu_hb->get_minimum_size().x), 0));
-               }
        }
 }

Done. And I undid the code I modified. Your code in EditorTitleBar::_notification is right. All I need to do is remove set_custom_minimum_sizes.

Thanks!