Whatever you have (in total) should be, in my opinion, at most 25-30% of usable storage you are to deploy.
For example, with six 4TB drives you are getting only around 12 TiB usable... Do you have more than 3TiB in total? I think yes. I would then consider either 8 drives 4TB each or 6 drives 6TB each.
The above assumes RAID-Z2. The difference between TB and TiB is difficult to get accustomed to, but one has to.
I just posted today elsewhere in the forum, that there are downsides to mixing drive types...
I haven't ever worked in a data center. But I heard about don't put a same batch of HDs in same box, you may get trouble if it's a bad batch, may fail at same time, or something like that, and different brands maybe a good idea.
you mention about HD troubleshoot, it that about same as what joeschmuck said in the same thread, to fix logicboard or motor failures. other than that, cannot tell any other HD issues can be fixed. but this' a factor should be considered, especially for small quantities, like 4 or 5 HDs, and not hard for handy people. I am handy or say tend to be handy, not afraid to open and replace parts, like computer hardwares troubleshooting.
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