the hooks “save_post…” are called too when a new post is created
then you can set default values with that
$customPostType = "customTypeCode";
add_action("save_post_$customPostType", function ($post_ID, $post, $update) {
if (!$update) {
// new item
// place to set default values
return;
}
// updating a existing item
// ...
}, 10, 3);