restic fell over hard at around 100 terabytes; the others at around 500.
I'm backing up a little over 3 petabytes. I use Bacula. It's awful, but I haven't found anything else that can deal with that kind of volume.
What ultimately worked? Plain old rsync over ssh, to a zfs pool with snapshots and compression.
As far as I could figure, the only notable downside of this is that the storage device must be trusted, since it has access to all of the data, and that you effectively needed root permissions on the storage you're copying to for filesystem permissions which makes a multi tenant backup server cumbersome (you could chroot or use containers or something but these solutions can become fiddly, e.g. running multiple instances of ssh on nonstandard ports to enable multitenancy).
EDIT: Ah I see rsync 3.0.0 (released in 2008) fixed this issue. Maybe I was on an old version.
But I also do backups to tape (LTO-8, in a Storagetek library), and I do like that Bacula handles that for me.
Did you test 'Burp'[1]?