If I was building the same system, I'd use 8GB of RAM and use the NVME drive for an L2ARC. I'd put the root fs into the main zfs dataset. The machine will be faster (~128GB of cache), cheaper and draw less power due to less RAM.

I'm not sure if TruNas supports this kind of config, or if they enforce booting from a separate drive. But FreeBSD itself is perfectly happy to boot from ZFS.