Uhhhmmmm, I think you missed this in my post above:
A cache drive just acts like a small buffer. For example, I have a 2TB cache drive, but I keep a lot of data on it, so I only have about 600 GB available for caching. If I am copying to my Movies share which has 5 TB free, I can only copy 600 GB before I get a disc full error, because the copy is to the cache drive, not the array.
If I want to copy more than 600 GB, then I would have to copy in multiple passes, i.e. copy 500-580 GB, stop, run the Mover, wait for the Mover to finish copying/moving to the array, then copy my next batch of data. This completely defeats the purpose of the cache drive (faster immediate copying of smaller amounts of data, overnight moving to the array when you're asleep), and only makes your job harder, so don't use a cache drive for large scale data migrations.
What Jamie is saying is that you can use Dolphin or Krusader to copy up to 3TB of data at once, since your smallest Unraid drive is 3TB. This assumes you are copying 3 TB of data directly to an empty 3TB drive.
If you are copying to the shares, then the math gets harder. You have to see which Unraid drive has the least amount of free space that is also more than the Minimum Free Space you set for that share. So for example, if you have 1 TB free on one of your disk1 3 TB, and your min free space is set to 150 GB, then using Krusader/Dolphin to copy to that Unraid share it will pick disk1 (since it has more than 150 GB free), but because you only have 1 TB free if you are trying to copy more than 1 TB you will get out of space errors.
You seem to have some preconceived notions about how Unraid works, probably based upon your experience with FreeNAS and other NAS products. Unraid is different and unique. While the benefits of Unraid are great, the design does make this particular data migration task harder than you would expect.
Long story short, you keep looking for a shortcut to copy a complete share from your FreeNAS to a single share in Unraid, and unless you use the right tool, this WILL NOT WORK. And to be clear, Krusader and Dolphin are NOT THE RIGHT TOOL for this. If you are willing to copy smaller amounts of data, directly to each disk, where you manually manage the amount of data copied to prevent from running out of space, then Krusader and Dolphin are fine.
The ONLY way to accomplish your goal of a FreeNAS share to Unraid Share copy in one pass is to use a tool that copies 1 folder at a time, just like Jamie stated, and doesn't create each folder until it copies it. To be clear, I don't know a tool that does this, other than that excerpted line of code I provided above from the Unraid Mover script, which processes each directory one by one.
And it is at this point I would recommend you turn to the larger Unraid forums community for more help on this. I've never done a data migration like this with Unraid. Jamie has, so some of what I know came from his own experience with his migration, plus my experience with having shares run out of disc space when I did not use them correctly. Many of the users on the Unraid forum are 10x smarter than I am when it comes to this stuff. They might be able to point you to a tool that will work correctly to copy the data the way you want. Or they might be able to help you customize that line of code I shared above, so that it will correctly copy the way you want.