Potential failure of Fusion drive in 2017 iMac

I believe you would be better served by getting an external SSD, clone your boot drive to it with Carbon Copy Cloner and booting and running from it. The labor cost to open and close the iMac will be more than the iMac is worth. And when you move on to a new Mac you can easily take the external SSD with you.

I recommend you contacting OWC's (MacSales.com) Customer Support to see which of their many units would best suit your particular Mac and budget. They are considered the premier 3rd party hardware supplier for Macs.

Apple won't replace the fusion drive with an SSD, only replace it. An Apple Authorized Service Provider will but a considerable price.

Peripheral point from a non-expert. I had a 2017 iMac 27-inch 18,3 and no problem with the Fusion drive apart from slowness. In January, I sought advice from the nearest Apple Authorized Service Provider as to whether a SSD upgrade was possible. I was told (quoting their words):

"It is quite a disassembly job to get to the Fusion drive inside but this is proving to be a popular upgrade lately to bring older machines more up to date and to offer better performance. It involves removing the mechanical hard drive part, and replacing with an SSD - usually 1TB but others are available - and pairing the small existing Flash memory part of the fusion combo with it."

My point is that their charge for this, "Apple parts" included, came to rather less than the Thunderbolt SSD external that I was considering, with a marvellous improvement in performance. Even if there is nothing wrong, I would recommend it so long as Apple support lasts for this machine.

That's why, if needed, we recommend getting an external SSD, clone the boot drive to it with Carbon Copy Cloner and boot and run from the external SSD. Read and write times will be approximately 10x what the fusion drive provides as described in this topic: imac spinning ball - Apple Community

First and foremost make sure you have a full and current backup.

To confirm that there's no software involved with the problem download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

like this:

Then we can evaluate the report to see if we can determine the cause of the problem.

Actual labor cost for my SSD replacement will depend on where you are. For me, in the UK, it was £99 including tax (say $114 at current rates), just under a couple of hours' work, reinstallation and data transfer included. (Having an idea of what was involved, I was impressed.) You have to intend to keep the Mac for some time to justify this. I should have said that the limiting factor remains the SATA connection, and speed does not go far above today's Etrecheck reading, though this is better than with the previous:

I do not doubt that the external SSD is recommended as a faster solution.