My iMac takes more than 5 minutes to start up

Well you have a mess on your hands. First is there is ZERO evidence this computer is backed up which means all or your data is at risk and there is a 100% guarantee you will lose data. At the very least you should be using Time Machine, if you are not aware of Time Machine please click Back up your Mac with Time Machine to learn. Using Time Machine will require an External Hard Disk, many experienced users prefer the OWC Mercury Elite Pro. In your case a 2TB model would be fine.

Also, I suspect the HD portion of your Fusion Drive may be dying. To test it please download and run DriveDX and test the HD, not the Fusion Drive and not the SSD, ONLY test the HD!!!!!!!!!!

If any errors appear that indicates the HD is failing and must be removed from the equation, another reason a Time Machine backup is critical.

The DriveDX report does show some errors that I don't like, indicating "Pre Fail" which means that the drive is past its optimum age and the drive could fail soon. A classic indicator of a HD beginning to fail is slow startup times, exactly what you are complaining about. So you can live with the HD failing and when it does fail by living without a true backup plan you will lose data. We can suggest that you backup and point you to the tools for doing so however if you don't want to follow that advice of course that is your prerogative and when the drive fails then you may have to learn the reason for backing up correctly the hard way. No judgement, just telling you that your current backup strategy is flawed and what may happen which is lost data.

However back to your HD, what many people do in your situation is to take the HD out of the equation by buying and using an external SSD as your startup drive. This is pretty simple to do if you are good at following directions, you can find those in How to setup an external SSD as your startup disk. Of course you will need to buy an external SSD, the one I (and many others) recommend is the OWC Envoy Pro SX due to it's high quality, reasonable cost and the vendor (www.macsales.com) specializes in Macs and everything they sell is extremely well tested and works great. They also have excellent pre and post sales support in the rare event you need questions answered or run into an issue. I'm not affiliated in any way with them, I am simply a long time (20 + years) customer that has been very pleased.

Best of luck!

FYI ... A "Fusion" drive is actually comprised of two drives; a smaller SSD & a much larger HDD. Via software they appear as a single drive. rkaufmann87 is referring to the HDD portion that should be checked for failures using DriveDx.

Should DriveDx report failures, that HDD should be replaced. In addition to a replacement drive, it will require "breaking" the Fusion Drive combo, and re-establishing it after replacing the HDD. This is something you can have either Apple or an Apple Authorized Service Provider perform for you.

You take the HD out of the equation by using an external SSD as your startup drive. You can learn how to do this by clicking How to setup an external SSD as your startup disk

Try resetting the NVRAM and SMC, or check for software updates. If the problem persists, consider checking your startup items or reinstalling macOS.