As far as I know Mint is still very much a lot of gnome-shell under the hood; They recently removed the dbus setting for that - which disabled a very usefull extension...
Maybe the line from my udev - controll backlight toppic works for you?
gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.freedesktop.DBus.Properties.Set org.gnome.SettingsDaemon.Power.Screen Brightness '<int32 30>'