Startup disk info is stored in NVRAM (PRAM). You didn't mention resetting it.
Ah, the one thing I didn't do….however now have. Here's what I did.
1. Shut down and cleared PRAM on restart.
2. Allowed restart to complete and turned on the backup drive. It appeared on the desktop.
3. Did a restart (assuming that would make sure the Mac knew the backup drive existed and was connected)
4. Shut down and restarted with the Option key depressed.
Unfortunately I still just got the main drive and the recovery volume.
Another point: Can you boot successfully from the SD backup?
Yes. I just can't get the iMac to understand, during startup, that it exists.
