I think the issue relates to UEFI Secure Boot locking the kernel.
This should be correctable by entering your BIOS and turning it off.
Step 1: Boot into the system settings by powering on the system and using the manufacture’s method to access the system settings.
Step 2: Look through the menu and select UEFI as the boot mode.
Note: Many menus show UEFI and Legacy as the choices, while others may display UEFI and BIOS.
Step 3: Navigate to the Secure Boot option and turn it OFF.
Note: On some devices, you should first boot once after disabling UEFI and go back to the settings menu to disable Secure Boot.
Thanks for performing the steps to turn off UEFI Secure Boot.
Thanks also for posting your latest log files. It seems that you stayed up for a couple of days before the system shut itself down.
Now I see the reason why. You have configured your system to perform unattended upgrades.
This leads to a situation where your computer is shutdown without your interaction in order to fully apply the upgrades.
The syslog.1 file you posted tells the story in its last two lines:
Jul 11 08:40:22 dev systemd[1]: Started Login Service.
Jul 11 08:40:22 dev systemd[1]: Started Unattended Upgrades Shutdown.
Personally, I like to see what upgrades are available at boot time and not automatically turn that over to an unattended process.
I suggest you read this article and decide whether you wish to turn off unattended upgrades. It's not for everyone.
How To Disable Unattended Upgrades On Ubuntu
I think this will end unexpected outages.
OK, so it wasn't the unattended upgrades.
I see something else I'd change, and that would be to login with Wayland as your display system instead of X. It's going to be the default Ubuntu display manager in the future, anyhow.
Inside of syslog, there seem to be some errors and warnings when opening up X Windows displays. It's old and some say reliable, but it has memory leaks, and may possibly be exposing you to network threats, but Wayland has been proving its worth and Canonical has been fine-tuning it.You just have to choose it when you login.
Look for the gear symbol in the lower right purple screen when you login, and choose Wayland.