Ayanefan

All good. Issue resolved!

Ok, testing complete, bulletpoints:
DO NOT use SMR drives as a parity drive. Things go well until a few Gigabytes get written then the constant read/write/check will kill the speed if copying directly to the array. This can be from 140MB/s then down to 2MB/s in my case. Switching to CMR has fixed that issue, the writes are consistent once the disks speeds from copying on the array saturate and slow the array down. I am surprised that this is not in BIG, BOLD, RED letters anywhere.
Your array will be only as fast as your slowest disk when accessed. So in my case, I have 2 parity CMR drives, 1 CMR in the Array and 5 SMR drives. When initially copying to the CMR, speed stays consistent around 120MB/s. When the disk hit the half-way full mark, it moved to the next SMR drive. After 20 minutes, the speed slowed down to around 20 - 40MB/s . Significant drop. Mine is a budget build using hardware I had on hand. I will replace the SMR drives with CMR slowly over time.
SMR drives are perfectly fine if you are using it as long term storage and do not need to do constant writes to them. Read times are fine, write actions are not.
Definitely use a NVMe drive or other fast sata drives to cache your copies, set your mover to run every night if you are not moving this over all the time. Use the system idle time to do things like trimming, moving or parity checks. If you are not there, you won't care.