That sounds like a job better suited to something like Percona's Xtrabackup. Use that to create the backup with no downtime, then copy the backup and any related partial/delta backups to a remote location.
I have 1.3 million posts and my database backup is 850MB.
One thing I suggest is to purchase Enhanced Search so you can empty your xf_search_index, that will reduce the size of your backup considerably. Transferring 850MB from my server to my house takes only a few minutes. However I have two hard disks in my server and I set up a Cron to copy the backup to the spare server every day.