Last thing I have to suggest is “which”, to see if there are multiple instances of python installed.
which python
If it only lists a singular entity, then retry the install commands with “–reinstall”, which might trigger something.
apt install --reinstall python3-all
apt install --reinstall ubuntu-desktop
Finally, give --fix-missing a shot if the above does not work.
sudo apt --fix-missing update
The hard part is determining where it broke.
I am hesitant to recommend “purging” the related packages using apt, as that is generally a more convoluted version of reinstalling. It is generally considered to be an advanced level procedure.
Booting from a live distribution you should be able to locate your hard drive and backup the data in “/home”. Do you have a USB hub to connect more than one USB device simultaneously?
It is possible to use the mount command on an external device from a command line via ssh in order to copy files over to a thumb drive, but that is probably going around your elbow by that point.