If you are doing any kind of writing, ANY kind, even just a few bytes any longer then a minute it will get inconsistent, especially as the drives fill up. With data in the write cache track the drives pretty well ignore you until that data is safely written to the SMR tracks, and if that means a rewrite of a track it will do it.
These are my system tunables that helped flatten out my transfers though:
ignore kmem size, and the l2arc settings if you dont have one. the arc_max depends on your ram size.
As well, you want to turn of any kind of of writing of last accessed updates to the array unless you need them.
Thats a setting on the host computers though.
This is my CIFS settings:
ea support = no
store dos attributes = no
map archive = no
map hidden = no
map readonly = no
map system = no
I also enabled readahead in the share properties, ideal for media servers as it reads a huge chunk then goes back to other things, gives the drives time for their house keeping and any write access.