Concerning the compatibility, there shouldn’t be a difference between LMDE and Debian. The choice of the Desktop environment depends on your personal favour. As Mac user, you probably tend towards Gnome which can be installed during the setup of Debian, not LMDE. However, as I don’t know about the performance of the 2013 iMac, maybe Cinnamon or Mate are a better choice.

anything that isnt very hard to run should be fine. ive personally ran a distro on an early 2012/2013 mac and it worked just fine. forgot what one but i know it was a very common mainstream one. i also somehow got kali on it so that was interesting too. if you want something easy and simple you should probably use mint or debian if they support imacs, otherwise? its really down to personal pref

I decided to use a rolling distro, in order to get the newest kernel drivers My favorite rolling distros are OpenSuse Tumbleweed and Arch OpenSuse TW was great out of box on my old iMac, but you most likely have to get some proprietary firmware in order for WiFi to work (see dmesg for drivers missing their FW) I decided to use arch (install using archinstall python script) from now on because I prefer the installation of community packages if the AUR using yay instead of searching https://software.opensuse.org/packages and click “one click imstall” download the file, double klick the file, which opens YaST frontend for repo management and then klick multiple times until the package is installed 😄

Pro of that behavior of openSuse is, that you don’t have to touch the terminal even once (except for checking dmesg to see if all driver work as intended)

Maybe, it makes sense to upgrade wifi using a dongle (you may have only wifi 4 in the iMac and there are usb dongles for wifi 6) or just use Ethernet.

I had problems using suspend on my iMac (screen was glitchy after wakeup) so I prevented systemd to trigger suspend and always turn the iMac off after usage. Command to disable suspend: sorry forgot but was something with simlinking

But anyway, now that I searched for it, I found this: apparently you can prevent the glitchy screen if you boot via legacy BIOS instead of EFI, to achieve this, you have to install your Linux (and thus grub2) from a liveOS booted from a cdRom instead from a stick (old mac boot efi from stick and legacy bios from usbstick) once you have your Linux booting using legacy bios, you can from now on boot your ISOs using grub and you don’t have to use any stick or CDrom ever again on that machine, as long as you leave have your boot partition untouched 😇

Feel free to ask more (I love when people try to make old hardware usable and prevent eWaste that way)