You can find an updated version of this guide using Instances on TrueNAS 25.04 in the Home Assistant Forums. I will update this guide when I have more time.
Do to the recent announcement - TrueNAS Virtualization Plans for 25.04.2 - the OP guide will not be updated since it is recommended to install new VMs using the Virtualization screens that will be restored in TrueNAS 25.04.2
The guide in the Home Assistant forums contains steps for using both Instances and Virtualization and will remain as such, however creating VMs using Instances will not be recommended.