This code will allow to select default author in General Settings. Available users aare those with role Author, Editor or Admin (cn be tweaked in code). If none i chosen setting ddefaults to user ID 1 – usual superadmin (also can be chaanged).

// Add the field to the general settings page
function itsg_add_author_setting_field() {
add_settings_field(
'default_post_author', // ID of the settings field
'Default Post Author', // Title of the field
'itsg_default_post_author_field_html', // Function to display the field
'general' // Page to add the field to (General Settings page)
);

// Register the setting
register_setting('general', 'default_post_author', array(
'type' => 'integer',
'description' => 'Default author ID for new posts',
'sanitize_callback' => 'absint', // Ensure the value is an absolute integer
'default' => 1 // Default value if not set
));
}
add_action('admin_init', 'itsg_add_author_setting_field');

// HTML for the settings field
function itsg_default_post_author_field_html() {
$value = get_option('default_post_author', 1); // Get the current value, default to 1

// Fetch users with 'Author', 'Editor', or 'Administrator' roles
$user_query = new WP_User_Query(array(
'role__in' => array('Author', 'Editor', 'Administrator'), // Array of roles
'orderby' => 'display_name',
'order' => 'ASC'
));

$users = $user_query->get_results();

// Create a dropdown list of users
echo '<select id="default_post_author" name="default_post_author">';
foreach ($users as $user) {
echo '<option value="' . esc_attr($user->ID) . '"' . selected($user->ID, $value, false) . '>' . esc_html($user->display_name) . '</option>';
}
echo '</select>';
}

// Set the default author for new posts
function itsg_force_post_author( $data, $postarr ) {
// Retrieve the default author ID from WordPress options, default to 1 if not set
$default_author_id = get_option('default_post_author', 1);

// Set the author for new posts
if (empty($postarr['ID'])) {
$data['post_author'] = $default_author_id;
}
return $data;
}
add_filter( 'wp_insert_post_data', 'itsg_force_post_author', 10, 2 );

// Set the revision author to the same author as the post
function itsg_set_revision_author($post_has_changed, $last_revision, $post) {
global $wpdb;

// Update the post_author of the revision to match the original post
$result = $wpdb->update(
$wpdb->posts,
array('post_author' => $post->post_author),
array('ID' => $last_revision->ID),
array('%d'),
array('%d')
);

// Basic error handling
if (false === $result) {
error_log('Failed to update revision author for post ID ' . $post->ID);
}

return $post_has_changed;
}
add_filter('wp_save_post_revision_check_for_changes', 'itsg_set_revision_author', 10, 3);