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Can you get the iMac to boot to ANY version of the OS?
Any one at all?
(by "boot" I mean the Mac boots, the user is able to log into the finder)

Fishrrman's "Mac Rule Number 1":
A Mac that boots (to any version of the OS) is preferable to one that won't boot at all.
 
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Can you get the iMac to boot to ANY version of the OS?
Any one at all?
(by "boot" I mean the Mac boots, the user is able to log into the finder)

Fishrrman's "Mac Rule Number 1":
A Mac that boots (to any version of the OS) is preferable to one that won't boot at all.
I did install the internal recovery of El Capitan on the external SSD. But updating from that is where I'm hitting the roadblock. I can log in and use it etc.
 
I did install the internal recovery of El Capitan on the external SSD. But updating from that is where I'm hitting the roadblock. I can log in and use it etc.

You might try creating a bootable installer of Monterey on to a USB drive and then using it to install itself on to the external drive from there. There are numerous instructions to do this on the Internet and here is Apple's:

I don't have your exact your situation so not certain it will work differently enough from the Internet Recovery approach, but I think it's worth a shot.

Also if that doesn't work, show us the full partition map of the external drive. If you can use Terminal share the output from:
diskutil list

Another thought: you may need to do this in two jumps: first go to like Mojave and then go to Monterey from there.
 
You might try creating a bootable installer of Monterey on to a USB drive and then using it to install itself on to the external drive from there. There are numerous instructions to do this on the Internet and here is Apple's:

I don't have your exact your situation so not certain it will work differently enough from the Internet Recovery approach, but I think it's worth a shot.

Also if that doesn't work, show us the full partition map of the external drive. If you can use Terminal share the output from:
diskutil list

Another thought: you may need to do this in two jumps: first go to like Mojave and then go to Monterey from there.

I tried going up incrementally instead of straight to Mojave. No luck.

Still get firmware error no matter which OS after El Capitan I try to upgrade from.
 
I tried going up incrementally instead of straight to Mojave. No luck.

Still get firmware error no matter which OS after El Capitan I try to upgrade from.

I would try the creating a USB installer and then using that to install Monterey onto the external disk. If it doesn't work, post the partition map of the external disk as configured by the USB installer.
 
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