MYSQL DATABASE BACKUP
700GB is too much for mysqldump. Restore will take too much time (but I would measure to know numbers). XtraBackup will be faster.

Taking backup from a slave is OK, but you have to make sure the master and slave are in sync. Use pt-table-checksum to verify that.

And what is missing in your strategy is backups verification. If you don't verify backups you don't have backups.