Wow. That sounds like a nice rig. Did you look at systemctl --failed and also look through sudo dmesg and see if there is any errors relating to the backlighting. On my HP i used this kernel parameter and i also disabled a service that shows up in dmesg related to amdgpu_b10
You could try the following kernel parameter also and see if it works. There are a number of these parameters for this and depends on manufacturer.
acpi_backlight=video
Edit: Don’t forget to run sudo grub-mkconfig -o /boot/grub/grub.cfg after adding the kernel parameter in the default grub command line.
Also here is the enable magic sysrq key