Replacing the SSD in a Fusion Drive

As Old Toad suggested, I'd recommend you use an external SSD as your new startup disk. An external SSD is very simple to set up, and the solution can be had for under $100. Plus, when you replace this Mac, you can keep using the external SSD for storage with your new Mac.

For more info, instructions, and what external SSD to buy, please see: How to Setup and Use an External SSD as your Startup Disk on a Desktop Mac - Apple Community.

Jack

What I would do is get an external SSD, clone your internal drive to it with Carbon Copy Cloner and boot and run from the external SSD. That way you'll save the labor cost of replacement and when you move on up to a new Mac you can take the eternal SSD with you.

replacing the SSD blade in most Macs is not a user friendly operation. but since you haven't mentioned which iMac you have, I can only recommend that you go to OWC’s website and search their DIY videos for your specific Mac to see if it can even be done, and if so, how to do it.

additionally, if you are going to be opening the Mac up anyways, I would recommend you also replace the HDD portion at the same time.

and of course make a backup of the Mac first.

Replacing the blade drive requires a complete teardown. It can be done, but it's a delicate operation that is not for the average user to even attempt.

Since you said this is essentially your key backup machine, I'd say it's time to make sure you have sufficient offline backups of your data and to replace the iMac.