Now that I have my NAS in full operation and am quite content with it, I thought it would be a good idea to make a full backup of the data again.
This is not as straight forward as I had hoped it to be.
ZFS does offer a possibility to send entire filesystems and incremental snapshots over the network. This is described nicely in the manual, perhaps with too little detail. In doing this I had suspected the network to be the limiting factor, and had already planned on improving my gigabit network with link aggregation - the bundling of multiple network ports into one connection.
But the actual doing shows the transmission into the network to be limiting. Very much so. Using the following command:

zfs send -R tank/public@may | ssh -C myuser@192.168.0.8 pfexec /sbin/zfs receive -dF matrix
I yielded a stunning total of 12MiB/s in transmission speed.

Sending about 4TB of data would take 90-100 hours which I considered to be not really acceptable.
To send the data a different way I now transfer the data to a separate 2TB harddisk, then transfer the harddisk to the other system and offload there. The advantage I see is that the systems don't have to run in parallel for such a prolonged duration. The commands I use for that are:

on the primary NAS machine
zpool create mypool c7t1d0
zfs send -R tank/public@may | zfs receive -dF mypool
zpool export mypool
on the secondary machine
zpool import mypool
zfs send -R mypool/public@may | zfs receive -dF matrix
zfs export mypool