Your jails won't be touched by a re installation of TrueNAS unless you somehow installed them on the boot device. Save a copy of your system config before you re install and then upload it after the installation.
If you created it manually (pure Jail, not from the "Plugin" catalog), you can also make a backup copy of this folder, found inside the Jail's filesystem: /var/db/qbittorrent/
The exact folder for those using the "Plugin" might be different.