We have encountered an issue with Apple's fusion drive, where if you work move masses amounts of files on a daily basis, the drive will come to a crawling halt. We have an identical iMac with SSD where this issue didn't happen. Replacing the Fusion Drive with an SSD - best move ever and a new lease of life for my iMac
Like any first time repair, your a little nervous especially when your using a tool to separate a screen from it's self, but the iMac cutter tool made this a lot easier than I feared and the guides were amazing!
Just buy the tools suggested and follow the video. Small investment which still comes out over half the price of an Apple SSD. Go ahead and replace the slower drive. You won't regret it! Just take your time on the repair, there is no rush. Just like cutting wood, measure twice cut once.
iMac Opening Wheel
iMac Service Wedge
iMac Intel 27" (2012-2017) Adhesive Strips