I use aws s3 deep archive storage class, $0.001 per GB per month. But your upload bandwidth really matters in this case, I only have a subset of the most important things backed up this way otherwise it would take months just to upload a single backup. Using rclone sync instead of just uploading the whole thing each time helps but you still have to get that first upload done somehow...

I have complicated system where:

• borgmatic backups happen daily, locally
• those backups are stored on a btrfs subvolume
• a python script will make a read-only snapshot of that volume once a week
• the snapshot is synced to s3 using rclone with --checksum --no-update-modtime
• once the upload is complete the btrfs snapshot is deleted

I've also set up encryption in rclone so that all the data is encrypted an unreadable by aws.