I have installed the newer version of Linux Mint 20.1 on another laptop and there, I can just change my screen brightness using
- Fn(Function button)+F1 for "brightness-" and
- Fn+F2 for "brightness+".
The functions of the F-keys are written on the keys anyway. Perhaps it is just working because they happen to be at the right F1/F2 place, since on my other laptop, they are at Fn+F11/F12. Another difference which might be part of the problem: at the other laptop, the function button is enabled as the standard.
I still guess that it just about updating Linux Mint to 20.1 to get this feature on Fn+F1/F2, so that it will not matter what is written on the keys. Whoever has the issue even on Linux Mint 20.1, please comment.