I’ve just successfully installed a 1TB SSD in place of my optical drive thanks to the information here- thank you to everyone who has contributed!
3 comments- the procedure described here seems to vary between HD replacement and dual HD/optical drive replacement. This can be confusing at times.
Expect there to be minor differences between the layout described and what you find when you open up your iMac. There are also, surprisingly, differences between the HD enclosure description, and the article provided by ifixit.
I was able to replace the DVD/HD enclosure single-handedly without fully removing the LCD, or deconnecting any of the cables (steps 5-11). It’s a bit cramped, and fiddly, but I was not happy removing any of the cables despite watching videos, etc., since they all seemed to involve applying more force than I was comfortable with. A more detailed description of how to release them might have helped, but even here, there may be minor variations even within the 2389 model.