i do run ashift 12 on my pool, mostly for hdd cross compatibility between 512B, 512E and 4Kn.
the special vdev is to to take blocks below 4K in size so running them at 4K, would mean they would get filled much much faster since the majority of the files are like 1Kb in size until they grow and is moved to the HDD’s instead.
also zfs actually writes 8K compression just makes it often fit in a 4k sector.
ashift just defines the minimum size of its sectors, it can also write 8k
the special vdevs can be individually configured outside the native pool ashift, for better capacity usage of the SSD’s
so i fully agree with what you say, i understand and have it handled…
my disk that is dying is just not made for what i’m using it for, plus i did some bad configurations for it and it has been doing a lot of internal writes handling its data which causes excessive wear.
the bad special vdev PCIe SSD is a 2012 design, so basically first gen PCIe based SSD.
my DC P3600 is still only at 1% wear after nearly a year, so its all good.
i have no unexpected write amplification.
pretty sure its mostly internal data being rewritten on the bad ssd due to me setting it to atomic writes… WOOPS 
and haven’t had a replacement nvme ssd to fix it.