Hey,
Looks like a proper built in my eyes, with a lot of potential to grow as you intend to do.
Just some remarks; currently RaidZ2 is not supported by Hexos, Hexos will always create a RaidZ1. You can manually create a RaidZ2 pool in Truenas but this might have unforeseen impacts and consequences.
You might want to consider buying a second boot drive and having the boot drives running as a mirror. This can save you a lot of headaches in case the boot drive fails, because right now, it's not possible to import pools into Hexos, meaning if the drive dies you cannot reinstall Hexos, because it will wipe your drives and even if pool import is supported in the future, it might still be worth the consideration, especially since you are already going full server build anyway.