As a long time ZFS admin, I have a few suggestions.
ZFS snapshots and send are much faster than rsync. Nearly all of them time is spent actually transferring data, and very little is spent enumerating data. One day it dawned on me that I could do hourly, or even 5 minute, snapshot && send on machines that could only handle daily rsyncs on ext4. It still depends on your write bandwidth and overwrite percentage, but it removes number of files from the equation.
Regarding vdev reorganization, it's true, you can't really change vdevs in an existing pool. I got around that by destroying the zpool on the backup server, re-creating it the way I wanted, then zfs sending the FS over again. The actual failover process is part of the manual failover setup anyway, so flipping cost me less than a minute of downtime. Let it burn in for a few days, then rebuild the original server's disks.
One last thing it took me a while to figure out. RAID-Z is faster than RAID10. Even for your IO bound processes, like PostgreSQL or MySQL. I'd done so many benchmarks showing that hardware RAID10 was better than hardware RAID5 for IO load, that I didn't even think about re-testing that conclusion under ZFS. Much later, I noticed that my storage servers (RAIDZ) could handle more IO than my database servers (RAID10). A 4 disk RAIDZ was faster than a 4 disk RAID10, and a 4 disk RAIDZ2 was the same speed. And I had 5 bays for spinners, so I could actually do a 5 disk RAIDZ vs a 4 disk RAID10 (8 bays total, including 3 for mirrored ZIL + L2ARC). As always, your benchmarks will vary. Just don't forget to re-test conventional wisdom.