This is for my reference I recently needed to revert the Kernel on my iMac/Linux machine. I had updated to the latest version, which caused a problem with the Brightness not working anymore. On an iMac that meant I had no way to adjust the screen, as it doesn’t have any physical buttons. I attempted to edit the grub file from the boot loader, but that didn’t work as there doesn’t seem to be an editor in the boot shell....
The default installation of Linux Mint on my iMac went very well right up until the moment it didn’t. I don’t know the cause, but I did find a few posts online about the problem I had, namely a login loop: whereby the login screen appeared to accept the login details only to show a blank screen momentarily, and then return to the login prompt. There were comments online proffering a solution in the form of ALT-CTRL-F1 etc....
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....
Sadly, Apple has retired my 2012 iMac, after 10 years. The iMac is still working, has a decent CPU and memory and a nice display, albeit not retina, so the retirement does feel somewhat premature to me. Apple puts things out to pasture far too quickly in my opinion, Microsoft is a lot better in this regard, and Linux is top. Due to concerns about the aging MacOS version on the machine, it limits my application upgrade path, how long will developers keep compiling for older systems 2 or 3 versions, maybe, often they want the latest and greatest version immediately, who can blame them....