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.
Same option, just went with Thunderbolt/usb3 unit. but I'll hook it up as thunderbolt.If you open up the iMac and -break something-, you're not going to like it.
MUCH MUCH easier to buy an external USB3 SSD, plug it into a USB port, and set it up to be an "external booter". It will be nearly as fast as an internally-installed SATA drive.