my hdd is showing 3 hdd same name, but non existant

I highly recommend you create a bootable macOS 11.x Big Sur USB installer now while you still can since that can give you more options for reinstalling macOS in case Internet Recovery Mode gives you problems.

Create a bootable installer for macOS - Apple Support

Next I would want to securely erase the internal Hard Drive portion of the Fusion Drive so the next owner won't be able to use a data recovery app to access any of your personal files since a basic erase with Disk Utility will leave the actual data on the old Hard Drive (you don't need to worry about the SSD since a simple erase will wipe its data due to how SSDs work). The Disk Utility erase won't show a "Secure Erase" or "Security Options" option when an SSD is selected.....it will only show up as an option when you are erasing the Hard Drive.

By securely erasing the Hard Drive you are protecting your data from being accessed and it allows you to recreate the Fusion Drive so you don't have to worry about any other partitions or volumes. Just make sure the Fusion Drive has the APFS file system if you are installing Big Sur.

How to fix a split Fusion Drive - Apple Support

FYI, there may be one or even 20 virtual volumes & Disk Images associated with the macOS installer on the left pane of Disk Utility. Ignore them since you cannot manipulate them & they won't be there when the system is booted normally to the internal Fusion Drive.

Edit: And before you erase the system, make sure you follow the instructions in the following Apple article to detach some of your accounts from this system:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If I boot into recovery mode with an option key there are 4 hard drives showing and I can't delete them.

It might help if you were to take a photograph of that and post it here so that we can see what you're seeing, and perhaps point you in the right direction.

I was hoping to rehome my iMac but it would be confusing for the users

Yes it would. However, we are discussing a Mac that is now ten years old, with limitations that will also be confusing for many users.

You're absolutely right in that it has been a brilliant machine and is likely to remain so for many years to come, but it is most decidedly obsolete. Personally I have never been a fan of "Fusion Drive", never bought a Mac with one, and it has now been well over a decade since I purchased any Mac incorporating any hard disk drive. If you are not averse to replacing it yourself, a solid state drive from OWC would be my recommendation. They have instructional videos so that you can determine for yourself whether it's worth the effort and expense. Start here: https://eshop.macsales.com/installvideos/imac_27in5k_14late/