shrink zfs root zpool

[Ultima]

Assuming you have done this before and understand everything well enough,
yes this would work and how I'v have done it in the past. Once you get the
process down, the usb drive will just be a backup and only a reboot would
be required. Also, a zpool labelclear maybe required on the disk you're
resizing.

Of course I'm sure you already understand this, there is also the period of
possible dataloss due to no redundancy. Because of this, I would also
suggest making backup, and also just in case a mistake is made to avoid any
possible chance of disaster.

Best of luck,
Ultima