gdm3 not starting on ubuntu 20.04

I recently upgraded from Ubuntu 18.04 to 20.04 as I was having this issue as well (with a different error). I was hoping upgrading would fix it. After upgrading Ubuntu worked a few times but now it won't start. I've been trying for ages to fix this including un-commenting #WaylandEnable=false, however when I tried that the laptop wouldn't start at all and stayed on the line dev/sda1: clean, xxxxxx/xxxxx files, xxxxxxx/xxxxxxx blocks and wouldn't go any further.

It might help to know this is a very old laptop >10years dell xps. Ubuntu works fine when it starts and is the only OS installed.

The output of systemctl status gdm is (I am typing this so might be errors):

Started GNOME Display Manager.
gdm-launch-environment [963]: pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
gdm3[958]: Gdm Display: Session never registered, failing
gdm3[958]: Child process -1011 was already dead.
gdm3[958]: Child process -1011 was already dead.
gdm-launch-environment [1324]: pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
gdm3[958]: Child process -1346 was already dead.

I would really like to get this fixed as it is a laptop I use a lot. I also really quickly tried to install lightdm but ubuntu told me there isn't enough disk space in /var/cache/apt/archives (which I am pretty sure is untrue last I checked I had about 9GB free) but I can delete some stuff and install lightdm if that is the best fix.

I created the file /usr/share/gnome-session/sessions/gdm-shell.session containing:

[GNOME Session]
Name=Computer

and then I could use GDM3 as the session login manager again for the first time in years.