problems upgrading my imac

Understand the below method will Wipe the Drive of ALL DATA and there is No Recovery - period

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Monterey would have to be done on a computer that Qualifies to run Monterey.

macOS Monterey is compatible with these computers

NOTATION >> There are reports appearing on the ASC Forums regarding failed installation of Monterey on older Qualifying Apple Computers. In most cases this is related to having a NON Apple Original internal drive ?  

The EFI Boot ROM included in Monterey is verifying for Original Apple Drive and if not found , may Balk or fail to upgrade to Monterey. 

Reports indicate the EFI BOOT ROM is Required to Flash the Computer and is a  requirement for a Successful Monterey Installation.

Once that is done, read on for preparing the Destination computer 

>> Only works on Intel Based Apple Computers.

Extra Special Notation regarding the Touch ID equipped Apple Computer.

About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map.

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

It appears that the drives in your iMac was initially setup as a Fusion drive using both the SSD and HDD together for speed at an affordable cost since at the time your iMac was designed SSDs were expensive.

See the following to determine if you have a Fusion drive:

How to fix a split Fusion Drive - Apple Support

SOme users have reported here problems with a Fusion drive.. HOwever, if you only use the HDD your Mac will be slow since typically you will have a 5400RPM HDD.

I would install on the Fusion drive is that is what you now have. After that I would backup and then replace the slow HDD with an SSD and install Monterey on that migrate data to the new Monterey.

Welcome and Thank you too.

Do come back to the ASC Forums, in the future, if / or when additional question may arise

As to your question - my learned colleague has answered the question. Follow that advice