I had a similar issue in that TechTool Pro was telling me my internal HD was going south on me, but Intech's SpeedTools said the drive was fine. Incidentally, I don't put much trust in what Disk First Aid has to say about any drive's health. It's a great utility for doing a lot of things, but it isn't worth a tinker's damn about the actual state of a hard disk. It's either a "go" or "no go" with Disk First Aid.
I'd do what Honestone recommends about your course of action. And as far as Carbon Copy Cloner is concerned, I think it's a superb product and maintain multiple clones of every Mac I own and can boot from any of them.
I don't envy you the prospect of having to replace the internal drive of your iMac, though. I finally got up the courage to do it myself. The biggest task isn't disassembling the thing as much as getting the display re-installed with all the micro-wiring and teenie-tiny little connectors plugged back in. Other World Computing has the suction cups and tools you'll need, in addition to free downloadable videos to guide you through every step if you undertake this endeavor.
And if you do, I heartily recommend you opt to go with a SSD. They're becoming very affordable, and they're unbelievably fast. In any event, you really should make an effort to keep no less than 10% of your drive free. The drive will last longer. (I keep at least 20% or more free, but I'm anal like that.) And if you go with a SSD, you'll never ever need to defragment, regardless of what you may read or hear. The wear leveling technology constantly re-writes data on the drive to enhance its longevity. Actually, defragmenting a SSD will greatly reduce its life span.
As far as defragmenting is concerned, I found this in one of OWC's old blogs:
Why Macs Usually Don’t Need to be Defragged
Since the days of Mac OS X “Panther” (which shipped in 2003), Mac OS X and its successor macOS have used something called Hot File Adaptive Clustering (HFC) to automatically defragment files. Think of HFC as constantly performing defragmentation on hard disk drives, ensuring that the HDD is always as optimized as possible for reading and writing data. HFC works very well for most users and as a result, most users should never need to defrag a Mac hard drive.
The one problem no one ever seems to address when installing a third party SSD in a Mac is that your machine won't have T.R.I.M. support enabled. (TRIM is enabled on factory SSDs, however. It just isn't when you pop in a replacement or upgrade.) The SSD will work fine, but it will die sooner. To enable T.R.I.M., you'll have to make some changes in the Terminal to turn it on. And as usual, I can't find the link at the moment, but it can be found at Other World Computing's website (macsales.com). If not, I'm confident one of the gurus running the site here can help you out.
As a side note, I do keep three clones for my 2010 iMac. One has El Capitan, one has Sierra and the last one has High Sierra, but I hate that OS. It's dog-poop slow on every Mac I own, and even though I've updated it religiously, I still don't trust it. And on my old iMac, El Capitan actually runs the fastest and is the most stable.
I really like my old 2010 iMac, even though it only has 8GB of RAM and doesn't have USB 3. At least it has FireWire 800 which I can tolerate, I like the optical drive and the built-in SD card reader is a real plus.
I have an older iMac (mid-2010) running High Sierra... I recently had an external drive fail and I decided to get a little more proactive with regards to what my other drives are doing. UI ran Diskwarrior on my iMacs internal drive and got an error message, not letting me defrag because of a drive failure (!) It said I should hurry and get my data off of it as soon as possible. I decided to run the mac's own Disk Utility and it said everything is fine. 
