Thanks a lot @le51, I confirm it works !
mkdir /zynthian/zynthian-my-data/scripts
create post install script:
nano /zynthian/zynthian-my-data/scripts/update_zynthian_sys.sh
and add the following:
#!/bin/bash
echo "# customized by /zynthian/zynthian-my-data/scripts/" >> /boot/config.txt
echo "[pi4]" >> /boot/config.txt
echo "over_voltage=5" >> /boot/config.txt
echo "arm_freq=1950" >> /boot/config.txt
echo "gpu_freq=250" >> /boot/config.txt
echo "framebuffer_width=1280" >> /boot/config.txt
echo "framebuffer_height=720" >> /boot/config.txt
echo "[all]" >> /boot/config.txt
Make it executable
chmod a+x /zynthian/zynthian-my-data/scripts/update_zynthian_sys.sh