I mentioned in the original post that I had reset NVRAM (twice, as it happens). I reset NVRAM a third time, and lo and behold, could now boot from the external SSD. Go figure.
iMac won’t boot from external SSD
I mentioned in the original post that I had reset NVRAM (twice, as it happens). I reset NVRAM a third time, and lo and behold, could now boot from the external SSD. Go figure.