Software modifications are no longer needed because pins_BTT_SKR_MINI_E3_V3_0.h has been modified accordingly.
SKR Mini E3 V2/V3 have 3.3V logic but LCD screen is adapted to 5V microcontrollers so appropriate modifications are needed.
Schematic of the 2004 LCD with the modification included:
To drop down the voltage from 5V to 3.3V you can use linear voltage regulator or simply a zener diode and a resistor like in the photo below.
Full mainboards pinouts you can find here. To make the required corrections, first rotate the connector on the LCD side. This can be done by gently levering up the plastic part and pulling it out.
Then change the connections on the ribbon cable as shown in the pictures below.
Pin A1 must be left alone to be connected to the middle pin of the Z-PROBE connector on the motherboard. DON'T DO IT BEFORE PCB MODIFICATIONS!
Now you are ready to use 2004 LCD! Before building the marlin make sure to comment CONTROLLER_WARNING("BTT_SKR_MINI_E3_V3_0", "ZONESTAR_LCD") in Marlin/src/pins/stm32g0/pins_BTT_SKR_MINI_E3_V3_0.h which warns you about 5V LCD module incompatibility.
LCD Mod: https://www.youtube.com/watch?v=ftRg0S-_qts
2004 LCD schematics: https://github.com/ralf-e/ANET-3D-Board-V1.0