This worked for me. It will probably work for you with minor touches if you insist. After I ran this for a while I didn't like how unreliable Duplicati was with the actual work of backing up and I began to investigate Duplicacy instead. I will leave this here for a year or so because it will probably remain more or less valid for that long and I did spend a lot of time putting it together to just thwack it because it's not useful to me. YMMV. GL. GG.
This is a note on installing Duplicati, a free Backup server into a TrueNAS Core jail.
In TrueNAS create a Jail running the latest FreeBSD image available (13.2-RELEASE-p4). I am calling this jail instance backup