Just recording this here as a reminder to myself. I tried out a couple of backup tools to run offsite backup of my TrueNAS Core ZFS system dataset pool and settled on Duplicacy, since it seemed reliable and works with BackBlaze B2. Setting up Duplicacy was as simple as downloading the command-line executable then executing it from /mnt/<system dataset pool name>, after which I following this guide to configure B2 and a two monthly executions of Duplicacy to perform the backup and prune aged-out snapshots. The switches that dis/enable the backup are in /etc/defaults/periodic.conf and are named weekly_backup_enabled and monthly_backup_enabled.