The separate cron container made the most sense to me. Other variants “work” but imo are mostly workarounds to avoid setting up a real cronjob. Beyond this I have no real reason, nor can I vouch that is is more or less performant than others.
Yes, the mysqldump container is for easier restores. It’s much easier to restore from a .sql file than a raw data dir that was copied while the DB was running ;) (speaking from experience…)