Yeah, iFixit is great. I've done some minor surgery on MacBooks to replace batteries or get cat hair out of them. I also put an SSD to my old Mac Mini. Honestly it is all sort of fun if you have the right tools and instructions at hand, and take it nice and slow.
But the whole "it's glued together" part of the 5K iMac sounds like a nightmare, and even assuming I didn't botch that somehow, the thing also has a pretty crusty old graphics card which is starting to show its slowness. I think I'm gonna go the coward's way out and sell it and pick up a refurbed one.
Just saying from someone with experience (me) opening both the previous model iMacs and these ones with the 'taped-on' screen ... these with the taped-on screen are actually easier IMO to open up and upgrade. Once you get past the anxiety of the unknown, cutting the tape (using the correct iFixit tool) and then reapplying the tape is no problem at all.
Additionally, unlike the 2011 and prior iMacs, these only have two different wires attaching the screen to the motherboard. The older models have 4 or more.
Bottom line, I feel like these are in fact the easiest iMacs in which to upgrade the HDD that Apple has ever produced.
P.S. That's why I will absolutely buy one of these 2019 models once I pass my 2017 iMac onto my daughter. Whatever model iMac Apple releases next will almost assuredly have a soldered-on SSD and will be prohibitively hard (impossible) to upgrade/replace.