Hey there!
Can you clarify something, because that text was a lot 😄
Have you got HexOS and TrueNAS Scale up and running now, albeit without any HDD pool?
Are you trying to use your existing 4x4tb HDD in HexOS
You mention RAID a whole lot, but hardware and software RAID. But HexOS and TrueNAS only support OpenZFS as far as I am aware, so is your existing pool in the OpenZFS format, or actually RAID?