Nice write up. My home ZFS is off pending two replacement drives because the hot spare failed during resilver. Oy vey.
... and yes - this is the problem with 'dedicated' SPARE drives - they just sit there unused and You never know if they will work when the time comes.
ZFS resolved that with DRAID where there are not 'dedicated' SPARE disks but 'spare block' spread across the disks:
- https://github.com/openzfs/zfs/wiki/dRAID-HOWTO/001b44728e1a...