I'd suggest Option 3 as the glued iMacs can be the very devil to work on.
Connect a USB3 or TB drive externally, clone the operating system etc from the internal, select it as the boot drive in System preferences > Startup Disk and away you go.
The only slight difficulty will be with operating system upgrades as it will update the interior drive first, and on the reboot will do the external. I used the internal as the backup drive.