Setup

Machine A:

• RAIDz1 takes care of single-disk failure
• ZFS doing regular snapshots
• Syncthing replicates the data off-site to Machine B

Machine B:

• RAIDz1 takes care of single-disk failure
• ZFS doing regular snapshots
• Syncthing receiving data from Machine A

Implications

• Any single-disk hardware failure on machine A or B results in no data loss
• Physical destruction of A won't affect B and the other way around
• Any accidentally deleted or changed file can be recovered from a previous snapshot
• Any ZFS corruption at A doesn't affect B because send/recv isn't used. The two filesystems are completely independent
• Any malicious data destruction on A can be recovered from B even if it's replicated via snapshot at B. The reverse is also true. A malicious actor would have to have root access on both A and B in order to destroy the data and the snapshots on both machines to prevent recovery
• Any data destruction caused by Syncthing can be recovered from snapshot at A or B