Here are some quick notes on how Big Tree Tech (BTT) say that their BTT Mini12864 should be configured for Marlin which is strangely locked away in a freakin' PDF: https://raw.githubusercontent.com/bigtreetech/BTT-Expansion-module/master/BTT%20Mini12864/BTT%20Mini12864%20V1.0User%20Manual.pdf
// At line 1980 at the time of writing, // uncomment the following line. #define FYSETC_MINI_12864_2_1
// Uncomment line 2203 (tow) #define NEOPIXEL_LED
// Specify 3 pixels #define NEOPIXEL_PIXELS 3
// Set brightness to 255 #define NEOPIXEL_BRIGHTNESS 255
// Search for u8g.setContrast(contrast); // should be in file ultralcd_DOGM.cpp // and change to the following: u8g.setContrast(255);