It was potentially harmful in 19.3 and below to use sudo with graphical applications as it could result in the app running as root changing ownership of files in your home to root. It's safer in Mint 20 as sudo no longer preserves the users home, but still not 100% free of potential trouble.
The admin protocol is a 'safe' way (albeit clunky) as is pkexec for apps with appropriate polkit rules. Personally I set my preferred text-editor as the sudo-editor and use sudoedit which is the safest way as the text editor itself never runs with elevated privileges
The kernel parameters are added to the grub config is the same way as you've done before.
I know you can use the gamma/brightness applet as a workaround but bear in mind if your aim for reducing brightness is saving power than the method that applet uses doesn't save power and it's not actually adjusting the backlight.
