Best strategy to backup btrfs root filesystem?

I have a Btrfs root partition with an @ root subvolume and an @home subvolume and I do auto-snapshots during updates and timeshift scheduled snapshots, both of which are saved on the same drive. This is great, but I want to have extra redundancy in case of a drive failure. In my last setup on Debian, I used the ext4 file system and put my timeshift rsync backups on an external drive. How can I do something similar, i.e. backup to an external drive, while still taking snapshots on the root device? In addition to the system device, which is a 1 TB SSD formatted as Btrfs, I have a 2 TB HDD currently formatted with two NTFS partitions since I dual boot Windows as well. Now, I would be willing to completely move to a Linux file system on that drive, but I don't know how I would handle backing up the root drive. I thought about doing a disk image onto the HDD with dd but if I do this, I would (a) loose an extra TB of storage if I understand correctly how dd works and (b) would not know how to restore from the image. Ideally, I would like to have a Btrfs partition on the second drive for backups of the root device only and a second (e.g. ext4 or NTFS) partition just for overflow data storage. Essentially, my question is: How can I facilitate a backup of my already "snapshotting" root partition (and also know how to restore from it)?