SATA 3TB HDD and NVMe SSD installed. Where is Mac OS installed?

Yes, the 3TB hard drive is in really bad condition. I hope you have a backup since it may not be possible to recover all the data on this drive as it has over four thousand bad sectors pending reallocation. I think the drive will need to be removed/replaced in order to use this iMac. I think you will find that this iMac will be very slow even booting from an external macOS drive due to the high number of bad blocks on the internal hard drive.

I don't see much that concerns me in your report.

There is this:

• System Software:

macOS Mojave 10.14.6 (18G6020)

Time since boot: About 72 days Time to reboot!

After you reboot, do a normal shutdown.

Then disconnect all of your peripheral devices and do another reboot into Safe mode.

How to use safe mode on your Mac - Apple Support 

Safe mode forces the OS to clear certain caches and logs and do other miscellaneous housekeeping. It's non-destructive to user data so is safe to do.  Very often certain weird software behavior is corrected by simply logging in this way as a troubleshooting method. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's normal.

Login to your user account, then exit safe mode by clicking Apple icon  > Shut Down....

Reconnect your peripherals and restart again normally.

With any luck, this will take care of your slowdown.

Disconnect your external drives in case one of them is causing the problem.

You can check the health of the hard drive (and SSD) by running DriveDx. While the report for the internal hard drive is most important you may as well post the report for the SSD as well. Post the complete DriveDx report here using the "Additional Text" icon which looks like a piece of paper.

That is a hard question to answer as some of depends on how you use the iMac and whether it is easy to replace the SSD on this iMac. I don't recall if the SSD on this iMac requires removing the Logic Board. Apple will only replace the original hard drive with another similar hard drive and will not perform any custom upgrades. Sometimes an AASP may perform some custom upgrades using third party drives.

If the internal hard drive does not interfere with an external boot drive, then using an external USB3 SSD would be the easiest option. However, with the number of bad blocks I'm concerned whether the failing internal will cause problems (if not now, then perhaps in the near future as the failure gets worse). There is a slim possibility you can erase the internal hard drive and create an "fstab" entry to prevent the internal hard drive from being mounted automatically when booting to an external SSD which may prevent or perhaps minimize the chances of the failing internal drive from interfering with an external boot drive.

It is difficult to open up an iMac and work on it as the Display Assembly is heavy & delicate and the cables are hard to see and connect as the connectors are really fragile & easily damaged. There is very little room for your hands. You can check out the repair/upgrade guides, but keep in mind those videos do make it look easy, but it is more difficult than it looks. OWC has some upgrade videos on their website.

If you are going to install a larger SSD to replace the failing hard drive, then that will give you two internal SSDs (not sure what you would do with the small original Apple 120GB SSD. Removing the failing hard drive and replacing the original Apple 120GB SSD with a larger & faster OWC Aura SSD is an option as well, but it may require removal of the Logic Board which is a major event increasing the risk of damaging the Logic Board or one of many fragile connectors. While an OWC will technically be significantly faster it depends on your work load & how you use the computer whether you will actually notice a difference in speed.

Here is the OWC page for SSD upgrade options for your iMac and the page also includes videos for installation (iMac 14,2):

https://eshop.macsales.com/shop/ssd/owc/imac-27-inch/2013-2019

Of course you can have Apple or an AASP replace the failing hard drive with another Apple hard drive so you can continue using a Fusion Drive setup. Keep in mind a hard drive will likely fail again at some point. While an SSD can fail at any time an SSD should last for a longer time.

And as I recall, 3 TB drives gad the absolute worst reliability reports.

macOS is normally installed on the volume named "Macintosh HD".

With a Fusion drive, that will be the HDD part of the drive. The SSD part is used by the OS to improve performance by buffering data transfer to/from the HDD.

If your mac is behaving slowly, it could be anything from a simple malware installation to a hardware failure. Some things are fairly easy to fix.

You should start with an EtreCheck evaluation of your Mac.

EtreCheck is a diagnostic app that gathers and reports important debugging information about your Macintosh. It is very highly regarded and meant to be used with Apple Support Communities to help others help you troubleshoot and resolve your Mac problems.

Please navigate to EtreCheck.com and download the free version of EtreCheck. Once you’ve run the app and created your report please post it with your reply to this message. 

You must upload the full report. For instructions on how to attach your EtreCheck report to your reply, please click  How to use Add Text when posting…

We can use the EtreCheck report to look for the more obvious things that may be causing your problem and advise how to correct them.