Here we are.
Report for the people trying to squeeze Free software onto Apple computers:
Ubuntu 20.04 LTS was able to boot and that's the reason we used it to Free this machine a little bit.
NOTE: Ventoy was not able to boot any damn distribution, stucking on a black screen, with a full white cursor pointer in the middle of the screen. You have to manually flash an USB device.
NOTE: Debian GNU/Linux buster 10 was not able to boot.
NOTE: We replaced the HDD with an SSD Crucial MX500 500GB but the installation failed twice because of strange I/O errors, like what happens with old HDD with hardware failures. This nonsense was fixed formatting again the SSD with gparted. I created a GPT partition table with an ext4 and the next installation went successfully. Absolutely don't know what happened.
NOTE: Now there is an unfriendly UEFI-related warning at startup probably spawned by the iMac's BIOS screaming at the sight of some Free software. Anyway it disappears after few seconds so I just decided to apply the don't care way.
WARNING: The Wi-Fi needs a proprietary Broadcom firmware to work.. Anyway it's automatically recognized by the Ubuntu's driver wizard.
WARNING: The NVIDIA video card needs a proprietary firmware to work but also without that support the video is good. Anyway it's automatically recognized by the Ubuntu's driver wizard.
WARNING: GNOME freezes when changing the user session. Maybe we can try replacing gdm with lightdm but untested.