On the 2019 Retina 5K iMac, the SATA power and data connectors are aligned one above the other, not side by side as in this image.

On the Early 2020 model at least, you will almost certainly have to loosen the logic board before you can do this step. This involves disconnecting the left speaker, and removing the two rightmost logic board screws. On the 2018 and 2019 models, this seems not to be necessary. If you have an early 2020 model and are trying these steps, don't force the USB-C connector out - loosen the logic board first. You don't have to remove it all the way.

Solved my own problem: I didn't have the battery connector pushed in far enough. Its edge really sits flush with the containing latch, though it isn't really obvious from the way it connects.

On boot after reinstallation, the fans go crazy during the Apple logo/progress bar, and the bar is very slow. Machine unuseable. I've reset SMC to no avail. Any other ideas?

Are these T8 screws security versions? If not, why does iFixit include a TR8 security driver in the kit?

I’ve added new details in the newest guide (Feb 2020)

If you have two original RAM sticks, they're installed in alternating slots, so there's no need to move anything around to install two more - just use the empty slots.