This isn't a limitation of TrueNAS, but rather of ZFS itself. A new ZFS feature that allows expanding the pool one drive at a time has been in the works for a long time now and is supposed to be rolling out really soon (as it has been for the past 2-3 years I think), but as of right now it can't be done on any NAS solution that uses ZFS.
As you noted, UnRAID can do it - because it uses a different kind of data+parity system (you can continue to add a drive at a time as long as the largest drive has your parity data, I think). If you were comfortable running TrueNAS in a VM then you should have no problem running UnRAID the same way. In fact, it will probably be faster just because it's less resource intensive (and certainly less RAM intensive than ZFS)