The machine is much easier to work on than it seems at first. You just need some good double-sided tape to put it back together.
I sure like my iMac with 8TB extra storage not to waste additional space on my desk and be connected via a fast and reliable SATA bus as opposed to USB-3 or super expensive Thunderbolt.
BTW, iMac 5Ks have been out for a few years now. Many of them are probably out of limited warranty anyway. $5k is quite a bit of stretch on the price approximation for an average iMac out there too.
In my experience, going to an Apple Store as opposed to an authorized repair shop is the way to go (I have had bad experience with the latter). Charming the "genius" on the other side of the bar is not a bad idea either. :)
I think that's all true. Also note that the external SATA SSD is going to be much slower (500 GB/s) than the internal (1700 GB/s or even higher).
This is one place the drawback of the Apple branded, unibody iMac strategy reveals itself. In a more roomy, user-friendly, conventional box case with standard parts, the consumer has a much more reasonably priced way to increase storage capacity without sacrificing speed or paying two times as much per GB.
and i think maybe Apple has squeezed component suppliers too hard. some components (e.g. power supplies) seem to be junkier than in the past.