Can't control backlight brightness on Xubuntu 20.04 on Lenovo Thinkpad L340-15IRH

The title says it all. The problem appeared when I fixed another problem. My laptop has an Intel Core i5 9300HF as cpu, the F means it has no integrated graphic chip and I guess that it’s for this reason that it sometimes booted stuck at 640×480 resolution (using the motherboard weak graphic chip).

I fixed this problem by booting in text mode. Uncommenting the famous GRUB_TERMINAL=console line. By the way, I looked for it but I have no idea why this fixed this, and maybe I could fix this another way and get the backlight back this way.

Anyway, after being able to start X on the graphics card and enjoy a comfy resolution, I couldn’t change the backlight brightness anymore. I tried a lot of things I found on SO and various websites: Adding some boot options such as acpi_backlight=vendor, acpi_osi=Linux, installing xbacklight (it never worked with any configuration), even echoing into /sys/class/backlight/brightness never changed the brightness.

Can I boot GRUB in graphical mode and still get X to use my graphics card ? If not, why ? And if not, how to fix the backlight ? What the heck is happening there ?

PS: setpci doesn’t work either