I watched the guy who replaced the hard drive when it failed in my 2011 iMac, and it didn't look very complicated. And though I, too, bought PC clones and modified them back in the day, I'd rather not do that now. And remember this came up to recommend swapping out the Fusion drive for an SSD.... so I just specified an SSD drive. I'm no fun, I know.
I ordered the 27" iMac with 8GB ram, the 3.4 CPU option, a 256GB SSD, and FCPX. I also ordered a Samsung T5 1TB external SSD, and an extra 32GB of memory.
Good job. That's the way to do it, get the performance you want, and guarantee no warranty problems.