Unraid "Array" disks can be formatted as ZFS but they exist as stand alone ZFS disks, they can't be RAIDZ. If you want to benefit from RAIDZ the disks need to be a pool, not part of the Unraid "Array". That means that they won't be protected by the parity disk so if you need protection for drive failure you need to have multiple disk in the pool and select the level of RAIDZ you want.
If you format your existing 8TB Unraid data drive to ZFS you will be able to take advantage of replication tasks from a ZFS pool. The existing 8TB drive data will still be protected by the parity disk if it is formatted as ZFS.
You should get about 110 MB/s over gigabit using SMB. If the share that you are writing to is set to Array Only instead of Cache Primary and Array Secondary then you will write slower because the data is going straight to the array disk and the server is doing parity calculations at the same time. Depending on the speed of your HDDs; to maximise your network transfer speed you might need a an SSD cache drive and set the share you are writing to as Cache primary then Array secondary. The data will go to the Cache drive first then the Mover task will move the files to the array drive at the end of the day or custom time that you configure it to run.
Watch and pay attention to these two videos to get a better understanding of how ZFS works in Unraid:
Spaceinvader One - How to Reformat (ZFS) and or Upgrade a Cache Pool in Unraid
Spaceinvader One - Array Disk Conversion to ZFS or Other Filesystems - No Data Loss, No Parity Break!