However on a Gameboy pocket I had to start with the custom logo rom instead of the nintendo logo rom and the delay is 500ms.

I'm using a purple Gameboy Color CGB-001.

That's strange indeed.

I'm using this simple code I wrote on Arduino and it works.

int ADR1 = 3; // A15 or A16
int ADR2 = 4; // A16 or A17


void setup() {

pinMode(ADR1, OUTPUT); // Pin 11 as Output
  pinMode(ADR2, OUTPUT); // Pin 12 as Output
  
  

  
digitalWrite(ADR1, HIGH);
 digitalWrite(ADR2, LOW);

delay(99);

digitalWrite(ADR1, LOW);
 digitalWrite(ADR2, LOW);

 
}

  void loop() {



  }

I'm gonna try the way of GB Dev FAQs as you stated to see if it works on a GBA.

From GB DEV FAQs:

I haven't tried this, but I guess it's the same for GBA.

I actually managed to boot a game with a custom boot logo on a Gameboy, since it reads this part twice, once for VRAM and once to check that it's correct.

So I've used an attiny85 to select the rom with the custom logo then a delay of 100ms then the rom to boot up, it works perfectly on a gameboy and gameboy color. (I've used arduino to program the chip)

However I couldn't find how to do it for a Gameboy Advance, the delay isn't the same (around 400ms) hence I still can't boot the game with the full custom logo.

I was wondering if it's possible on a GBA.

Regards