Well, you actually attach the SSD to the exiting boot pool.
3 - remove the external SSD, which is now my spare boot/OS and stick it somewhere safe
Yes. And once you’ve done that you can then remove the “attached” but not removed SSD from the boot pool. It will be marked as “REMOVED” but still present. If you boot with the 2nd SSD installed, it will update the mirror.
The issue is that it will annoy you by saying your boot pool is degraded whilstever the the other SSD is “missing”
BUT you do know that you could just save your config file as well. You can always re-install truenas, and upload your config.
So maybe just keep the mirror installed?
Also, you can make multiple boot environments, ie you can clone your current environmenet at any time.
4 - if the existing boot SSD fails, power down and fit the “spare” instead.
Yeah. Of course… you could also leave you “spare” in place…
Lots of options really.
(Separately I have just watched your superb video “Setting up and using Tiered Snapshots” and I had no idea that the functionality could be set up in such an effective way. I shall start looking at (e.g.) NILFS2 for implementing the Windows shadow copy approach which you showed, but in Linux Mint).