The project at https://github.com/swkim01/waveshare-dtoverlays offers only the new driver setup for the display, it doesn't provide any scripts.
can you elaborate?
Sure. You want to handle 2 video configurations - output to HDMI (which is default) and via the LCD. Save the default config.txt in /boot/ as /boot/config-hdmi.txt. Install and configure the LCD drivers (modify the config.txt) accordingly, make sure it works, then copy the config.txt to /boot/config-waveshare.txt.
Create 2 scripts in /home/pi/RetroPie/retropiemenuhdmi.sh, containing #!/usr/bin/env bash cp -f /boot/config-hdmi.txt /boot/config.txt lcd.sh, containing #!/usr/bin/env bash cp -f /boot/config-waveshare.txt /boot/config.txt make the scripts executable by running chmod +x /home/pi/RetroPie/retropiemenu/hdmi.sh chmod +x /home/pi/RetroPie/retropiemenu/lcd.sh
Now, when starting EmulationStation, you'll have 2 new entries in the RetroPie system - HDMI and LCD, executing one of them will switch the video output configuration. After running one of the entries, you'll have to reboot to activate the new configuration.