That last paragraph sums up the ZFS defrag procedure at one shop I worked at. Buy new disks and send/receive the pool. At our size and use case the timing was

That last paragraph sums up the ZFS defrag procedure at one shop I worked at. Buy new disks and send/receive the pool. At our size and use case the timing was usually close to perfect. The pools were getting close to full and fragmented as larger disks became inexpensive.