Similar question exists but the solution (using mv) is awful because in this case it works as "copy, then remove" rather than pure "move".

So, I created a pool:

zpool create tank /dev/loop0

and rsynced my data from another storage in there directly so that my data is now in /tank.

zfs list
NAME      USED  AVAIL  REFER  MOUNTPOINT
tank      591G  2.10T   591G  /tank

Now I've realized that I need my data to be in a child filesystem, not in /tank filesystem directly.

So how do I move or rename the existing root filesystem so that it becomes a child within the pool?

Simple rename won't work:

zfs rename tank tank/mydata
cannot rename to 'tank/mydata': datasets must be within same pool

(Btw, why does it complain the datasets are not within same pool when if fact I only have one pool?)

I know there are solutions that involve copying all the data (mv, or sending the whole dataset to another device and back), but shouldn't there be a simple elegant way?

Just noting that I do not care of snapshots at this stage (there are none yet to care of).