Help needed with Ubuntu 20.04 on iMac late 2009

I thought installing Ubuntu to recycle obsolete iMacs would be simple, it seems a little bit more than that...

So I've been tasked to refurbish a late 2009 iMac to give it out to some non-profit. I couldn't obviously give it out with the latest supported macOS for this model as 10.13.6 doesn't receive security update anymore and alternative web browsers (Firefox, Chrome...) will also give up with 10.13 sooner or later. The problem was even more serious with another early 2008 iMac limited to 10.11.

So I decided to install Ubuntu 20.04 on those iMac, and noticed some troubles with the 2009 model (iMac10,1 - Core 2 Duo 3GHz, 8GB RAM, Nvidia Geforce 9400, SSD 250GB + 2TB HDD) : start a few apps and it freezes (that's especially true when launching LibreOffice on top of Firefox). The mouse is still moving but nothing reacts to any click. It usually un-freezes after a minute or so, just to freeze again a few minutes later and so on.

This happens when trying Ubuntu from the live CD, after EFI or BIOS/Bootcamp install. I tried to switch to nvidia drivers using the "additional drivers" panel but the install fails and the system won't boot anymore after the failed driver install.

The 2008 iMac seems to be fine with its Ubuntu EFI install, including after updates. The 2009 failing iMac was still in use a year ago running 10.13.6 and never caused any trouble.

Any help would be greatly appreciated.

Jose

I finally found the solution here : Lost macbook pro LCD backlight brightness controls after installing nvidia driver

In a few words :

sudo add-apt-repository ppa:kelebek333/nvidia-legacy
sudo apt update
sudo apt install nvidia-340
sudo reboot

Then :

sudo gedit /usr/share/X11/xorg.conf.d/10-nvidia-brightness.conf

Where you need to paste :

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GEFORCE 9400M"
    Option         "RegistryDwords" "EnableBrightnessControl=1"
EndSection

Save and reboot.

Expect minor graphic glitches, but it should be usable.