Hello @gabyimagely, thanks for your reply.
I have added
define('NGG_CRON_SCHEDULE', 2592000); // 30 days
in my wp-config.php and in the WP dashboard, cron control panel, the ngg_delete_expired_transients has been re-scheduled with this delay.
Let’s see now if it remains as it is, also when the plugin is updated, because before this attempt any effort to change the schedule or delete the cron were overwritten shortly after.