A wild-ass guess on a possible help forward:
sudo apt purge nividia-driver-535 Enter
I'd expect this to remove most if not all of the driver and it's related packages. When all is removed you have "a clean slate" to start trying with another driver.
Check that all has been removed with dpkg -l | grep -i nvidia, if anything still has ii in the leftmost column, then purge it too (as above).
As no nvidia packages remain, try a reboot; I'd expect the "nouveau" driver to be active again.
In that state, feel free to try either 535 once again, maybe there was a mishap the first time - that prevented it to install correctly...
OR try an older one, which might work.
Repeat the purge if it still fails to boot into the GUI.
NOTE: there are "open" and "server" versions of the driver and also those that have no such designation (maybe call these "desktop"), avoid at least "server" - unless it IS a server.
And last but not least; that tutorial is "good", 'OK' to use that method to install the driver.
But: If you write down the driver name and use sudo apt install _driver_name_ you will see a lot more information. Among that information you will most likely have some kind of indication on WHAT the problem is, if there is a problem.
