The latest version of TrueNAS, TrueNAS Scale Electric Eel which is now in Beta supports RaidZ expansion.
There are some caveats, for best data usage you will want to rebalance your data, but that would allow you to add extra disks to your existing RaidZ2 vdev.
Thus, if I were you, I’d start by upgrading to TrueNAS Core 13.0, and then begin you migration to TrueNAS Scale Dragon Fish (24.04), and by the time you finish that, you should be able to migrate to Electric Eel in October, when the new expansion feature is released.
