I am currently in the process of resetting an old 2012 iMac after deciding trying to set up the Fusion Drive in there was too much hassle. I think it can be done but as it is a proprietary RAID setup I decided to reinstall MacOS and then use an external drive as a dual boot Linux/Mac setup. I had this before and it worked well enough, the only requirement was to hit the boot drive selector shortcut.
After a few false starts with the boot install set up and recovery to Mountain Lion, the drive was locked and not available for the install. I had to use the Terminal in the shell to reformat and mount the 1TB drive for install, easy enough once you know how, like most things.
I now need to update Mountain Lion to a version that can accept updates and browse the Web with HTTPS, so many things get broken if the system is 10 years old. At one point that it complained that the El Capitan DMG downloaded from Mac Store wasn’t valid, their advice download from the App Store, a infinite loop there I think.
My fault for not just dual booting the system in beginning instead of wiping the drive/s for a clean Linux install. All good though as I love breaking/dismantling things, this is how I learn when I get them fixed again, yea sometime they don’t get fixed, but that’s life.