Converting iCoud backup to Time Machine

Not possible. iCloud is not a backup solution for any Mac.

You'll need to secure an external hdd or ssd, connect it directly to your old iMac, format it properly and allow Time Machine to complete a proper backup of the iMac to the drive. Once that is done, connect the Time Machine backup drive to the new Mac mini before you turn it on for the first time to set it up. When prompted, you can then migrate your files from the TM backup drive to the new Mac mini.

To connect an external drive with a USB-A type plug to the new Mac mini, you'll need a USB-A to USB-C adapter. They can be found on Amazon and at other retailers for about $8 each. They look like this:

For additional guidance, please see:

Back up your Mac with Time Machine - Apple Support

Types of disks you can use with Time Machine on Mac - Apple Support

I have an additional comment that you should know. iCloud is NOT a backup plan for Macs. It is designed to allow for cross platform sharing, not backup.