Backup is not a tool. Backup is not a software you can buy. Backup is a strategy you need to study and implement to be able to solve your specific problems.
Many Linux users are asking how we’re migrating to FreeBSD, how jails can replace lxc or (in part) Docker, and how we’re monitoring and performing backups/restores.
We started a complex, continuous and not always linear operation, that is to migrate, where possible, most of the servers from Linux to FreeBSD. Here's why.
If you are using Proxmox, ZFS is one of the best local storage options. You can easily backup your lxc containers using Borg.
I've been using BTRFS for years and I've had few problems. Some of them are small and can be avoided with some smart improvements.