Brightness always at 100% after reboot

It's very easy, so if you would like to try it out, you might not have to wait (or might experience annoyance if it does not work).
You can add or remove a grub parameter by adding an entry or removing an entry from the Grub File.

sudo nano /etc/default/grub

Arrow key down to the parameters:
You can see two parameters already included in the quote marks: quiet and splash.
Add the parameter acpi_backlight=vendor inside the quote marks:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
Now tap ctl+o to overwrite, then tap enter to save as current configuration. Tap ctl+x` to exit the editor.
Now, you must update grub for the changes to take effect.

sudo update-grub

That is all there is to it. The parameter I am suggesting here is a Generic Parameter and may not be the right one you need. You can test it, in case it works but if not, we can try refining it. I will be away for many hours today.