thank you very much for your quick support, it really helped me.

It didn't work in my "big" program, but I wrote a small test program, and it fully meets the requirements:

#include <M5StamPLC.h>       

void my_set_backlight(bool o) {
  PI4IOE5V6408_Class p;
  p.setDirection(7, true);  // false input, true output
  p.digitalWrite(7, !o);    // false on, true off
}

void printAndWait(String s) {
  M5StamPLC.Display.println(" " + s);
  delay(4000);
}

void setup() {
  Serial.begin(115200);
  M5StamPLC.begin(); 
  M5StamPLC.Display.setTextSize(3);
}

void loop() {
  M5StamPLC.Display.fillScreen(TFT_BLACK);
  M5StamPLC.Display.setCursor(0, 10);
  printAndWait("Test 1");
  printAndWait("Test 2");
  my_set_backlight(false);
  printAndWait("Test 3");
  my_set_backlight(true);
  printAndWait("Test 4");
}

You can see it not only in the dark screen, but also in the power consumption, which is reduced by about 20%.

I suspect that in my "big" program, someone is turning the lights back on (and working against me).

Best regards,
Uwe

below is an alternative function which works for me too:

#define PI4IOE5V6408_ADDR 0x43
#define OUTPUT_REG 0x05

void my_set_backlight(bool o) {
  uint8_t data = M5.In_I2C.readRegister8(PI4IOE5V6408_ADDR, OUTPUT_REG, 200000);
  if(o == true) data |= 0b10000000;
  else data &= ~0b10000000;
  M5.In_I2C.writeRegister8(PI4IOE5V6408_ADDR, OUTPUT_REG, data, 200000);
}

Thanks
Felix

your my_set_backlight() function works for me and my_set_backlight(true) turns the backlight off (inverted logic).

void my_set_backlight(bool o) {
  PI4IOE5V6408_Class p;
  // false input, true output
  p.setDirection(7, true);
  // false off, true on ?
  p.digitalWrite(7, o);
}
void setup()
{
  auto cfg = M5.config();
  cfg.serial_baudrate = 115200;
  M5.begin(cfg);
  M5StamPLC.begin();
}
void loop()
{
  M5.update();
  if(M5.BtnA.wasClicked() == true)
  {
    Serial.println("BtnA clicked - backlight on");
    my_set_backlight(false);
  }
  if(M5.BtnC.wasClicked() == true)
  {
    Serial.println("BtnC clicked - backlight off");
    my_set_backlight(true);
  }
}

Thanks
Felix

P.S. the AI generated code did not work for me.

#include <M5StamPLC.h>
#include <Wire.h>

#define PI4IOE5V6408_ADDR 0x43  // I2C-address
#define DIR_REG 0x06                 
#define OUTPUT_REG 0x02      

void setup() {
  M5StamPLC.begin();  // Initialisiert I2C-Bus (G15=SCL, G13=SDA)
  
  // P7 (LCD_BL) is output, Set Bit 7 of DIR_REG = 0
  Wire.beginTransmission(PI4IOE5V6408_ADDR);
  Wire.write(DIR_REG);
  Wire.write(0x7F);  // 0b01111111 (only P7=output)
  Wire.endTransmission();
}

void setBacklight(bool on) {
  Wire.beginTransmission(PI4IOE5V6408_ADDR);
  Wire.write(OUTPUT_REG);
  Wire.write(on ? 0xFF : 0x7F);  // P7=1 (on) or 0 (off)
  Wire.endTransmission();
}