I tried clean install of Windows on my iMac a few months ago using Microsoft media creation tool and the latest version of Windows 10 and it did not work. I got constant reboot loop during Windows setup. With the below steps I am able to do a clean install of Windows 10 on iMac into the internal hard disk without any dual boot now.

1. I am using an older version of Windows 10 ISO from Microsoft, Win10_22H2_English_x64v1.iso.
2. Create Windows bootable USB using Rufus.
3. Unplug all external devices and keep only wired keyboard and mouse attached to the iMac.
4. Reboot and hold Alt key on startup. This will show the USB drive, which will mostly be the last one. Select it to boot from it.
5. Follow along the Windows 10 setup wizard. Delete all partition setup, create new ones and install Windows on to the largest HDD instead of SSD if you have a fusion drive. Installing Windows to SSD with 121 GB would be a problem in terms of space down the line.
6. Once the setup copies all files and goes for a reboot, remove the USB. This will make the system boot from the hard disk. The setup will continue from there and then Windows simply works.

Enjoy the freedom.