I’m not sure why, but it seems like TrueNAS in a VM is not recommended (I saw a thread on their forum)… I also wanted the ability to add more drives later on, so I’ve gone with UnRAID and even though it’s only been a few weeks, it seems pretty functional and I’m glad I paid the licensing fee. I am trying to do Proxmox and OPNsense on one of those fanless N305 boxes and am getting very confused!
You could consider a couple of those SuperMicro server cases and link one as a JBOD via a SAS card… see things like https://www.servethehome.com/sas-expanders-diy-cheap-low-cost-jbod-enclosures-raid/ I picked up a 12bay CSE-826 with a SAS3 backplane and two 80 Platinum redundant power supplies for $300. I know you’d need a larger one, but eBay is a great place :-)