Wanted to say thanks for this, very useful advice for a super niche case most people probably won't (and shouldn't) really need!
In my own use-case I had an awkward juggling match to deal with; when I got my four bay box, in an ideal world I would have got four disks and just setup a RAIDz2 on day one (for half capacity with two disk redundancy and maximum read performance not a concern) but because of the disk prices at the time it wasn't within my budget to get four smaller drives, biggers ones were better value but I could only get two so had to go for a basic mirrored pair.
Fast forward to when I can afford another two and my choice was either add another mirrored pair and just accept single disk (guaranteed) redundancy, or use what your guide showed. In my case I created a RAIDz2 with two disks and two sparse files, offlined both files, copied my data into the degraded pool, then replaced the sparse files with the disks from the original mirror one at a time.
While doing it this way meant there were technically always two full copies it was still pretty hair-raising, but my offsite backup doesn't keep as many historic snapshots so I really wanted to preserve them if I could.
