First of all I do agree with @megavolt that you should test your use cases in a virtualbox first before doing it for real.
Secondly, I am answering here as a long term zfs user. Although manjaro architect is supporting the installation on a root zfs I would not recommend to do that for a simple reason: desaster recovery.
You can not find any official live ISO image that would allow you to boot your system and mount the zfs root to fix things. Therefore, for the root filesystem I would always use an officially supported filesystem. That is the reason why my root is on XFS. But you can pick any other filesystem, like btrfs.
But all other filesystems on my PC are on zfs. And that includes /home. And some of the datasets are encrypted, natively with zfs. This is something btrfs can not do. btrfs either encrypts a full partition with LUKS or single folders with tools like gocryptfs.
LUKS is fine, but not flexible enough for me. It does not allow to only encrypt single folders. gocryptfs can do that. But it is really slow compared to native zfs encryption - very slow.
For external backup drives a filesystem with checksum support like btrfs or zfs make total sense. And btrfs+LUKS is ok for such a setup. Although I would always prefer zfs for various other reasons 