That's it! I can't use the 2TB HDD, but I was able to install an SSD with minimal effort and risk without removing the display, disconnecting the display cables, or removing the speaker, or HDD.

As a bonus, the HDD versions of the iMac come as "Fusion Drives" so the have a PCIe SSD attached as well. Mine had a 128GB SSD installed so I do have access to that as an extra drive now. I plan to install Windows on the PCIe drive via Boot Camp.