But I can tell you that unfortunately TurnKey installer isn't compatible with UEFI (nor secure boot). I won't go into the reasons why, but it is something that we hope to address for our next major release (although I can't promise it'll make it).

As it turns out, FreeNAS uses bhyve for hosting VMs. TBH, I've never heard of it until now and have 0 experience with it (or with BSD in general).

So the only thing I can suggest is that you see if there is a way that you can provide a legacy BIOS to your VM at boot time. TBH, I'd be surprised if there wasn't some option somewhere to do that (although TBH, I'm also surprised that it doesn't just default to legacy BIOS as most other hypervisors seem to).

If you're super happy with your FreeNAS setup, then this next option probably isn't appealing and/or relevant. So please feel free to ignore. But anyway I can't help myself... :)

Personally, (as a Debian lover) I'm a huge fan of Proxmox. One of the great things about it is that it can host full VMs (e.g. FreeNAS, Windows, etc) and/or containers via LXC (LinuX Containers). It's perhaps also worth noting that all the LXC builds of the TurnKey appliances are available for download within the Proxmox WebUI. LXC only supports Linux (so no FreeNAS, Windows, etc), but you get (near enough to) bare metal performance (as opposed to the overhead of a "full" VM). I know that BSD has a similar feature to LXC called "Jails", although as per LXC, it only supports BSD guests, so not much use for TurnKey...