Make of this what you will but I tried to replace my 2015 iMac's Fusion drive with an SSD using the OWC kit. I knew this would be a delicate operation so watched the video several times before even starting and went super slow and step-by-step. Got the drive in and tested it was working but the display would not turn on. Tried replacing all of the display cords one-by-one and nothing worked.
Brought the machine to the local Mac repair place who had 30+ years experience and he tried for *over a year* to fix it and could not get it working. After all that time I had to just leave it at that and say goodbye.
Thankfully my dad had an extra late 2014 iMac with a 4TB Fusion drive which is my music machine now. It is just starting to crap out on recent DAW plugins but up until recently it has been working fine. I have all my projects and audio instruments on separate SSDs which helps. I also doubled the RAM to 32GB a couple weeks ago and this has breathed new life into the machine and probably given me an extra 6–9 months.
All of this is to say, is it worth bricking your machine to do the upgrade? If it were me I would just buy a Mac Mini.
Counterpoint: I did this on my 2014 iMac 5K and ran into no issues whatsoever. Used a kit from iFixit and an SSD I got off Amazon. It's a doable operation. I'm certainly not the only one to do it, and I'm not some kind of genius -- I just followed the directions very slowly and carefully. Not saying you didn't, but clearly something went wrong or maybe there was a separate hardware failure that happened. (As a side note, I don't know what kind of Mac shop spends over a year trying to fix anything, but that might be part of your issue right there.)
In the end, I got a couple more years out of that iMac by putting a 2TB SATA SSD in place of the OEM HDD that had come with it as part of the Fusion Drive (which was then dying). I did not attempt to replace the blade SSD inside the iMac, as that's apparently much trickier to source and replace.
Running off that SATA SSD felt somewhat faster than the Fusion Drive, but not night and day. Don't forget, the whole idea of the Fusion Drive is that what you're actively working on is kept on a very speedy blade SSD, not the spinning HDD. It was a super clever hack for its time, back when SSDs were ungodly expensive. That said, the whole setup made them pretty prone to wearing out. Two points of failure, and one of them is a very tiny SSD that's getting hammered constantly with read/write cycles.
