Read and Write slow RAIDz1- truenas scale

Not an expert on controllers but it was deemed unsuitable here:

You would need an HBA in it mode like an LSI. But I think you will not be able to fit the drives in your case then? Does it use a backplane? Maybe someone else has a recommendation here.

Way too expensive $ per GB wise. 8TB+ drives are much more efficient.

Even your nvme speeds are slow, are you copying from a single HDD?