The hook wpmu_new_blog gives you a deprecated notice and tells you to use wp_insert_site. Don’t do that, wp_insert_site fires too early. Instead, use wp_initialize_site with a priority higher than 100, as in:

add_action( 'wp_initialize_site', 'wpdocs_action_wp_initialize_site', 900 );

/**
 * Fires when a site's initialization routine should be executed.
 *
 * @param WP_Site $new_site New site object.
 */
function wpdocs_action_wp_initialize_site( WP_Site $new_site ) : void {
	switch_to_blog( $new_site->blog_id );

	// do the job

	restore_current_blog();
}